Synergistically reinforced flame-retardant polystyrene / nylon composite film, preparation method and application thereof

Abstract

Synergistically enhanced flame-retardant polystyrene / nylon composite film and preparation method and application, the composite film is mainly composed of surface carboxyl modified polystyrene-based microspheres, nylon organic solution and epoxy crosslinking agent organic solution composite crosslinking.The preparation method is as follows: (1) interface pre-composite dispersion: in the heating stirring nylon organic solution, surface carboxyl modified polystyrene-based microspheres are added in batches, after the addition is completed, continue to react; (2) crosslinking reaction: slowly add epoxy crosslinking agent organic solution, after the addition is completed, continue to react; (3) film forming and post-treatment: flow into the mold, stand for shaping, after curing, take out, dry, demoulding, it is finished.The application of the composite film is also disclosed.The interface compatibility of polystyrene and nylon in the composite film is good, the mechanical strength is excellent, the flame retardancy is outstanding, and the thermal stability is good.The process of the application is simple, efficient, convenient to operate, low in cost and suitable for industrial production.

Description

Technical Field

[0001] This invention relates to a nylon composite film and its preparation method, specifically to a synergistically reinforced flame-retardant polystyrene / nylon composite film, its preparation method, and its applications. Background Technology

[0002] Polystyrene (PS) is a low-cost thermoplastic polymer with excellent processing properties. It possesses good insulation, water resistance, and chemical resistance, and is widely used in construction engineering, packaging materials, and electronic devices. However, polystyrene also has significant drawbacks, such as high brittleness, poor heat resistance, and extreme flammability, which greatly limits its application in high-end fields.

[0003] Nylon (polyamide, PA) is an important class of engineering plastics, renowned for its excellent mechanical strength, toughness, wear resistance, and heat resistance. Aliphatic nylons, such as nylon 6 and nylon 66, in particular, possess amide bonds in their molecular chains that can form intermolecular hydrogen bonds, endowing the materials with excellent comprehensive properties.

[0004] To obtain materials with superior overall performance, researchers have attempted to blend polystyrene with nylon, aiming to combine the low cost and easy processability of polystyrene with the high strength and toughness of nylon. While some attempts have been made to combine polystyrene and nylon in existing technologies, these methods and their effects have many limitations.

[0005] CN107936550A discloses a polystyrene / nylon 6 composite material and its preparation method. The method employs in-situ polymerization technology, using caprolactam and styrene as monomers, along with compatibilizer monomers and various initiators, to prepare the composite material through a stepwise polymerization reaction. While this method can improve the compatibility between the two phases to some extent, resulting in materials with low water absorption and high impact toughness, its process is complex, involving multiple heating and vacuum distillation processes, and demanding reaction conditions. Precise control of free radical polymerization and anionic polymerization is required, placing high demands on equipment and making large-scale industrial production difficult.

[0006] CN107434891A discloses a nylon / polystyrene alloy microsphere, its preparation method, and its application. This method uses porous nylon microspheres as a carrier to disperse polystyrene within their pores, but the molding process, such as selective laser sintering, is costly and unsuitable for large-area film preparation. CN106380833A discloses a nylon-styrene-based polymer-filled modified composite powder, its preparation method, and its application. This method aims to reduce costs and improve mechanical properties by preparing nylon-styrene-based polymer-filled modified composite powder for laser sintering. However, this method also focuses on powder sintering and requires the addition of large amounts of fillers and flow aids, resulting in numerous process steps that cannot be directly applied to solution casting.

[0007] In addition, although there have been studies on the preparation of polystyrene / nylon 6 flame-retardant composites by adding flame retardants, traditional additive flame retardants often have poor compatibility with the matrix. While improving flame retardancy, they may significantly degrade the mechanical properties of the material, such as increasing brittleness, making it difficult to achieve a balance between high strength and high flame retardancy.

[0008] In summary, although existing technologies offer various approaches to combining polystyrene and nylon, they all suffer from the following common and unresolved technical shortcomings: poor interfacial compatibility, difficulty in balancing high strength and high flame retardancy, the contradiction between process complexity and industrial production feasibility, and limited application targets. Summary of the Invention

[0009] The technical problem to be solved by the present invention is to overcome the above-mentioned defects of the prior art and provide a flame-retardant polystyrene / nylon composite film and its application with good interfacial compatibility between polystyrene and nylon, excellent mechanical strength, outstanding flame retardancy, and good thermal stability.

[0010] The further technical problem to be solved by the present invention is to overcome the above-mentioned defects of the prior art and provide a method for preparing a synergistically reinforced flame-retardant polystyrene / nylon composite film that is simple, efficient, easy to operate, low in cost, and suitable for industrial production.

[0011] The technical solution adopted by the present invention to solve its technical problem is as follows: a synergistically reinforced flame-retardant polystyrene / nylon composite film is mainly composed of polystyrene microspheres with surface carboxylation modification, nylon organic solution and epoxy crosslinking agent organic solution.

[0012] The inventive concept and principle of this invention are as follows: Utilizing an epoxy crosslinking agent to simultaneously react with the carboxyl groups of surface-carboxylated polystyrene microspheres with nylon end groups and surface-modified polystyrene microspheres with introduced active carboxyl groups, a unique three-dimensional interpenetrating network structure of "nylon-crosslinking agent-microsphere" is constructed. This is a fundamental solution from the perspective of interfacial chemical bonding, fundamentally different from simple physical blending or the addition of compatibilizers. The epoxy group (a three-membered ring, -CH-(O)-CH-) in the epoxy crosslinking agent molecule exhibits high reactivity due to its ring strain. When this ring structure is opened, the opened epoxy group reacts with the amino group (-NH2) at the end of the nylon molecular chain to generate secondary amines and hydroxyl groups. It can also react with the carboxyl group (-COOH) at the end of the nylon chain to generate ester groups and hydroxyl groups. More importantly, the opened epoxy group undergoes esterification with the carboxyl group (-COOH) introduced by the surface-carboxylated polystyrene microspheres, generating ester bonds and hydroxyl groups. A crosslinking agent molecule has two or more epoxy groups, so it can act like a "bridge", connecting one end of the nylon molecular chain and the other end to the surface of the polystyrene microspheres. Countless such "bridges" are interconnected, eventually forming a three-dimensional interpenetrating network structure of "nylon-crosslinking agent-microspheres" with microspheres as crosslinking points and nylon molecular chains as the network.

[0013] Preferably, the weight parts of each component of the synergistically reinforced flame-retardant polystyrene / nylon composite film are: 0.5-15.0 parts of surface-modified polystyrene-based microspheres, 30-100 parts of nylon organic solution, and 0.1-7.0 parts of epoxy crosslinking agent organic solution. If the amount of surface-modified polystyrene-based microspheres is too small, the content of microspheres in the matrix will be too low, making it difficult to form a continuous reinforcing phase and flame-retardant network. The mechanical and flame-retardant reinforcement effects will not be significant, and the advantages of the present invention will be difficult to demonstrate. If the amount of surface-modified polystyrene-based microspheres is too large, the microspheres are prone to agglomeration due to excessive content, which will destroy the material uniformity and cause the viscosity of the system to increase sharply, making it difficult to cast the film, and the film material is prone to defects, and the cost will also increase accordingly. If the amount of nylon organic solution is too small, the solid content of the solution will be too high, and the viscosity will be too high, making it difficult to cast and form a continuous and uniform film. If the amount of nylon organic solution is too large, the solution will be too dilute. Although it is easy to process, a large amount of solvent needs to be evaporated, resulting in low production efficiency, high energy consumption, and difficulty in preparing film materials with practical thickness. Epoxy crosslinking agents need to react with a sufficient number of carboxyl groups on the surface of microspheres to build a "bridge" between the nylon matrix and the microspheres, forming a preliminary three-dimensional network structure. If the amount of epoxy crosslinking agent is too small, there will be insufficient crosslinking points, making it difficult to form a continuous and effective crosslinking network, resulting in weak interfacial reinforcement and poor performance. If the amount of epoxy crosslinking agent is too large, the excessive amount of epoxy crosslinking agent will lead to an excessively high crosslinking density in the system, thereby causing a series of problems such as increased material brittleness, decreased elongation at break, processing difficulties, and high costs.

[0014] Preferably, the weight parts of each component of the synergistically reinforced flame-retardant polystyrene / nylon composite film are: 1-8 parts of surface-modified polystyrene-based microspheres, 80-100 parts of nylon organic solution, and 1-5 parts of epoxy crosslinking agent organic solution.

[0015] Preferably, the method for preparing the surface carboxylated polystyrene microspheres is as follows: after drying the polystyrene microspheres, an acid anhydride modifier is added and mixed, followed by air jet pulverization to obtain surface carboxylated polystyrene microspheres.

[0016] Preferably, the drying temperature is 60–120 °C (more preferably 80–110 °C) and the drying time is 4–12 h (more preferably 6–10 h).

[0017] Preferably, the amount of the anhydride modifier is equivalent to 1-9% (more preferably 1.2-8.0%) of the mass of the polystyrene microspheres. This amount ensures the optimal balance between modification efficiency and effect. If the amount of modifier is insufficient, the microsphere surface cannot be adequately covered to reach the threshold for introducing sufficient carboxyl active sites, resulting in insufficient interfacial modification; if the modifier is excessive, it will lead to critical issues such as multilayer physical adsorption, increased cost, and potential interfacial side effects.

[0018] Preferably, the D of the polystyrene-based microspheres 50 The particle size is 10–500 nm (more preferably 50–400 nm).

[0019] Preferably, the airflow pulverization temperature is 40–100 °C (more preferably 60–90 °C), the pressure is 0.5–0.8 MPa (more preferably 0.55–0.75 MPa), and the time is 0.1–0.5 h (more preferably 0.2–0.4 h), until the D of the surface carboxylated polystyrene microspheres is... 50The particle size is 100–800 nm (more preferably 300–700 nm). Under air jet milling, polystyrene microspheres are not only ultra-finely broken down by the mechanical energy of high-speed collisions, but more importantly, surface chemical bonds are broken, generating highly reactive free radicals. Simultaneously, the anhydride modifier is activated and undergoes an in-situ solid-phase reaction with the newly formed surface, covalently grafting carboxyl groups, thereby achieving in-situ solid-phase carboxylation modification and introducing active carboxyl groups onto its surface. The specified temperature and pressure ensure the activation energy required for the reaction, preventing the polystyrene microspheres from softening and sticking together, guaranteeing the solid-phase reaction, and providing the minimum energy for breaking down and activation, avoiding over-breaking, thus being economical and energy-saving. The specified time ensures complete and thorough air jet milling, preventing over-processing or efficiency reduction. The nanoscale particles obtained by airflow fragmentation provide a huge specific surface area, which can be grafted with high-density carboxyl groups, laying the foundation for the subsequent construction of a robust "nylon-crosslinking agent-microsphere" three-dimensional network. This is a prerequisite for achieving high strength and high flame retardancy. At the same time, this size range is much smaller than the critical size that causes macroscopic defects in nylon composite films, which can effectively avoid stress concentration caused by excessive particle size, ensuring that the composite film structure is dense and the performance is stable, and is also beneficial to the film formation process.

[0020] Preferably, the polystyrene-based microspheres include one or more of polystyrene hollow microspheres, polystyrene-based carbonized microspheres, or phosphorus- and nitrogen-containing flame-retardant modified polystyrene microspheres. More preferably, the polystyrene-based microspheres include polystyrene-based carbonized microspheres. Even more preferably, the polystyrene-based carbonized microspheres are derived from a polystyrene-based carbonized microsphere and its preparation method and application disclosed in CN110922787A, which is a reinforcing filler independently developed by the applicant. The method of this invention utilizes the advantages of polystyrene-based microspheres, such as high reinforcement, high filling, high temperature resistance, and high flame retardancy. After surface carboxylation modification, nylon is chemically and physically modified to obtain a flame-retardant polystyrene / nylon composite film with high strength, good thermal stability, strong flame retardancy, and excellent comprehensive balanced performance based on interface modification and crosslinking synergistic enhancement.

[0021] Preferably, the anhydride modifier includes one or more of pyromellitic dianhydride, maleic anhydride, succinic anhydride, or phthalic anhydride. More preferably, the anhydride modifier includes pyromellitic dianhydride and / or maleic anhydride. Utilizing the high activity, lack of byproducts, and high functionality of anhydride modifiers, they are perfectly suited to the core innovative process of "in-situ solid-phase modification via airflow milling".

[0022] Preferably, the nylon organic solution is prepared by mixing and dissolving nylon and an organic solvent at a mass ratio of 1:4 to 12 (more preferably 1:6 to 10). If there is too much organic solution, the mass transfer effect will not be achieved; if there is too little organic solution, the reaction is prone to agglomeration, resulting in defects such as incomplete cross-linking reaction.

[0023] Preferably, the mixing and dissolving temperature is 30–78 °C (more preferably 40–70 °C), the rotation speed is 50–100 r / min (more preferably 60–85 r / min), and the time is 0.5–4.0 h (more preferably 1–3 h).

[0024] Preferably, the nylon includes one or more of aliphatic nylon, aromatic nylon, copolymer nylon, specially modified nylon, or bio-based nylon. The aliphatic nylon includes one or more of PA6, PA66, PA610, PA612, PA11, PA12, PA46, or PA1010. The aromatic nylon includes one or more of PA6T, PA9T, or nylon MXD6. The copolymer nylon includes PA6 / 66 and / or PA6 / 12. The specially modified nylon includes one or more of reinforced nylon, flame-retardant nylon, or transparent nylon. The bio-based nylon includes PA510. The nylon used in this invention is in powder or granule form.

[0025] Preferably, the organic solvent in the nylon organic solution includes one or more of formic acid, trifluoroacetic acid, hexafluoroisopropanol, m-cresol, calcium chloride / ethanol solution, ionic liquid, phenol / tetrachloroethane mixture, dimethyl sulfoxide, or N-methylpyrrolidone. The ionic liquid includes 1-butyl-3-methylimidazolium chloride, etc. The mass concentration of the calcium chloride / ethanol solution is 3-10%. The volume ratio of the phenol / tetrachloroethane mixture is 0.1-0.4:1.

[0026] Preferably, the mass concentration of the epoxy crosslinking agent organic solution is 2-10%. This mass concentration range is the optimal window for achieving a uniform and controllable crosslinking reaction. If the concentration is too low, the reaction will be incomplete and the film formation will be poor; if the concentration is too high, it will cause local gelation and damage the material properties.

[0027] Preferably, the epoxy crosslinking agent includes one or more of aliphatic glycidyl ethers, aromatic glycidyl ethers, alicyclic or polyamine epoxy compounds, etc. The aromatic glycidyl ethers include resorcinol diglycidyl ether, etc.

[0028] Preferably, the organic solvent in the epoxy crosslinking agent organic solution includes one or more of methanol, ethanol, isopropanol, acetone, butanone (MEK), tetrahydrofuran (THF), dimethylformamide (DMF), or ethyl acetate. The solvent is miscible with water and compatible with nylon organic solutions. More preferably, the organic solvent in the epoxy crosslinking agent organic solution includes one or more of methanol, ethanol, acetone, or butanone.

[0029] The technical solution adopted by the present invention to further solve its technical problem is as follows: a method for preparing a synergistically reinforced flame-retardant polystyrene / nylon composite film, comprising the following steps: (1) Interfacial precomposite dispersion: In a heated and stirred nylon organic solution, polystyrene microspheres with carboxylation modification on the surface are added in batches. After the addition is completed, the reaction continues to obtain an interfacial precomposite dispersion. (2) Crosslinking reaction: In the pre-composite dispersion obtained by heating and stirring in step (1), an epoxy crosslinking agent organic solution is slowly added. After the addition is completed, the reaction continues to obtain a crosslinking composite solution. (3) Film formation and post-treatment: The cross-linked composite liquid obtained in step (2) is cast into the mold, left to stand and solidify, and then the pre-shaped film is cured together with the mold, taken out, dried, and demolded to obtain a synergistically reinforced flame-retardant polystyrene / nylon composite film.

[0030] Preferably, in step (1), the heating and stirring temperature is 30–78 °C (more preferably 45–72 °C), and the rotation speed is 60–150 r / min (more preferably 80–120 r / min). Under the action of mechanical shear force, the surface carboxylated polystyrene microsphere clusters are opened and uniformly dispersed into the nylon organic solution; a large number of hydrogen bonds will be formed between the abundant carboxyl groups (-COOH) on the surface of the modified microspheres and the amide bonds (-NH-CO-) on the nylon molecular chains, which is beneficial for cross-linking. Within the temperature range, the system can be ensured to have sufficient molecular thermal motion capability, promote solvent wetting, microsphere dispersion, and hydrogen bond formation, and also prevent excessive evaporation of the solution and excessive concentration changes; within the rotation speed range, the minimum stirring intensity can be provided to overcome the gravity of the microspheres, prevent sedimentation, and generate effective shear force to break up the agglomerates. At the same time, excessively high rotation speeds are avoided to prevent the introduction of too many air bubbles and the potential mechanical damage to the nylon molecular chains or the surface of the modified microspheres due to excessive shear force.

[0031] Preferably, in step (1), the batch addition refers to adding the material in three batches at a mass ratio of 4-6:2-4:1-3 (more preferably 4.5-5.5:2.5-3.5:1.5-2.8). Adding more material first and then less gradually guides the increase in system viscosity, ensuring that the stirring paddle generates sufficient shear force to disperse the material. Batch addition also improves the degree of mixing reactivity, meaning that the carboxyl groups on the microsphere surface and the amide bonds of the nylon molecules achieve "pre-composite" efficiency and sufficiency through interactions such as hydrogen bonds.

[0032] Preferably, in step (1), the total feeding time is controlled to be 10-30 min (more preferably 15-25 min). Under effective mechanical stirring, the time range is sufficient to complete the batch and stable addition of materials. If the feeding time is too short, it is equivalent to adding a large number of microspheres quickly at once, which will lead to excessively high local concentration of microspheres in the system. They will not have time to disperse and will form permanent agglomerates that are difficult to disperse, which will seriously affect the uniformity and performance of the final product. If the feeding time is extended too much, it will not only have little effect on improving the dispersion quality, but will also unnecessarily extend the entire production cycle and reduce efficiency.

[0033] Preferably, in step (1), after the feeding is completed, the temperature and rotation speed are maintained for 1 to 6 hours (more preferably 2 to 4 hours) to continue the reaction. Continuing the reaction within the above time range can ensure the completion of deep dispersion and pre-composite, balance performance and cost, and avoid over-processing.

[0034] Preferably, in step (2), the heating and stirring temperature is 30–78 °C (more preferably 45–75 °C), and the rotation speed is 80–180 r / min (more preferably 85–170 r / min). This step involves a crucial multi-component chemical crosslinking reaction, the core of which is that the epoxy crosslinking agent acts as a "molecular bridge," reacting chemically with both components to form a strong covalent bond. If the temperature is below 30 °C, the ring-opening reaction rate of the epoxy groups is too slow, and the crosslinking reaction cannot proceed effectively within a reasonable time, which may lead to insufficient crosslinking degree and an imperfect network structure. If the temperature is too high, it will approach or exceed the boiling point of the solvent, causing the solvent to evaporate violently, changing the system concentration, affecting the film quality, and even causing safety hazards. It may also trigger side reactions, such as the hydrolysis of nylon molecular chains (especially in the presence of trace amounts of water) or the homopolymerization of the crosslinking agent itself. A suitable rotation speed allows epoxy crosslinking agents to be dispersed instantly throughout the reaction system, preventing excessively high local concentrations that could form gel particles. It also facilitates full contact and mass transfer of the reactants. However, excessive rotation speed will draw in a large amount of air, forming bubbles. These bubbles will become defects in the subsequent film-forming stage, degrading the film's performance. Furthermore, it will exert excessive shear force on brittle microsphere structures (especially hollow or carbonized microspheres), potentially causing them to break and destroying their functionality.

[0035] Preferably, in step (2), the dripping rate of the slow addition is 20 to 40 drops / min.

[0036] Preferably, in step (2), after the feeding is completed, the temperature and rotation speed are maintained for 4–12 h (more preferably 5–10 h) to continue the reaction. The crosslinking reaction is carried out gradually, requiring sufficient time for the crosslinking agent molecules to diffuse, collide, and react with the active sites, thereby forming a complete network structure. If the reaction time is too short, the degree of crosslinking will be insufficient, the network structure will be weak, and it will be difficult to significantly improve the performance; if the reaction time is too long, for epoxy crosslinking systems, although the main reaction has been completed under prolonged heating, small side reactions may occur, resulting in a slight and continuous increase in crosslinking density, which may make the material gradually brittle, and will lead to reduced production efficiency and increased costs, which does not meet the requirements of industrial production.

[0037] Preferably, in step (3), the temperature for static setting is 28–38 °C, and the time is 0.4–4.0 h (more preferably 1–3 h). During the static setting film formation process, the main processes are solvent evaporation and the initial arrangement and fixation of molecular chains. This is a physical curing process, which controls the physical evaporation rate, prevents surface skinning and internal defects, and ensures the integrity of the film. If the temperature is too low, the slow evaporation may cause component migration or phase separation. Components with different densities or surface energies (such as microspheres) may have enough time to undergo uneven sedimentation or floating in the liquid state for a long time, which will destroy the uniformity of the membrane composition. Moreover, the solvent evaporation rate is too slow, resulting in low production efficiency and excessively long production cycles, which does not meet the needs of industrialization. If the temperature is too high, it will lead to the formation of surface skin and internal defects: if the solvent evaporates from the surface too quickly, the polymer chains on the surface will quickly concentrate and solidify, forming a dense "hard skin". After the internal solvent is heated and vaporized, it is difficult to penetrate this dense skin, which severely hinders the evaporation of the internal solvent. This will form irreversible defects such as bubbles, voids or pinholes inside the membrane, completely destroying the integrity of the membrane and causing its mechanical properties and barrier properties to drop sharply. In addition, the huge difference in evaporation rate between the surface and the interior will lead to uneven shrinkage and generate huge internal stress, which may cause the membrane to warp or crack during shaping or subsequent processing. If the time is too short, the membrane will still be a viscous fluid or have too low strength, which will cause it to flow, deform or break when the mold is moved, resulting in insufficient mechanical strength; if the time is too long, it will affect production efficiency and process control.

[0038] Preferably, in step (3), the curing temperature is 30–40 °C, and the time is 2–24 h (more preferably 6–18 h). The curing process is the deepening of the cross-linking reaction and the final fixation of the microstructure. It is a process involving both physical and chemical changes. During this process, the completion of residual cross-linking reactions, the orientation and relaxation of molecular chains, the deep removal of residual solvents, and the final stabilization of the phase structure occur. Simply put, curing is a "maturation" process, the purpose of which is to allow the chemical and physical structures of the material to reach their optimal and most stable state under a mild thermal environment. 30–40 °C is a "thermal annealing" window, which provides energy far exceeding room temperature to promote structural optimization, while being far below the critical temperature (Tc) that may cause defects. g (And solvent boiling point), to prevent the formation of defects such as bubbles and warping, ensuring that the curing process is carried out under absolutely safe and controllable conditions, ultimately resulting in a composite film with stable dimensions, no internal defects, and excellent performance. If the time is too short, the curing will be insufficient, and the material properties may continue to change slowly during use; if the time is too long, it will affect performance and production efficiency.

[0039] Preferably, in step (3), the drying temperature is 40–60 °C (more preferably 45–55 °C), and the time is 6–12 h (more preferably 6–10 h). If the drying temperature is too high or the time is too long, the nylon chain segments will become too mobile, causing the material to creep and deform (such as sagging or warping) under its own weight or small internal stress, making it difficult to maintain perfect flatness.

[0040] Preferably, in step (3), the curing and drying process is carried out in a vacuum environment or under an inert atmosphere.

[0041] Preferably, the vacuum level of the vacuum environment is -0.08 to -0.10 MPa.

[0042] Preferably, the inert atmosphere includes one or more of nitrogen, argon, or xenon.

[0043] The technical solution adopted by this invention to further solve its technical problem is as follows: the application of a synergistically reinforced flame-retardant polystyrene / nylon composite film, which is applied to the fields of electrical component encapsulation or insulation materials, high-performance lithium battery separators, flexible circuit boards, lightweight interior materials for rail transit or aerospace, or special safety protective clothing. The electrical components include flexible electronic devices, etc.

[0044] The beneficial effects of the method of the present invention are as follows: (1) The flame-retardant polystyrene / nylon composite film of the present invention contains polystyrene microspheres with carboxyl groups on the surface. Through interface engineering design and chemical crosslinking synergistic modification, the microspheres and the nylon matrix are firmly bonded to the chemical crosslinking network constructed by epoxy crosslinking agent. The tensile strength can reach 53.68 MPa, the elongation at break can reach 228.14%, and the Young's modulus can reach 758.14 MPa, indicating that it has good interfacial compatibility and excellent mechanical strength. The oxygen index can reach 34.32%, the glass transition temperature is ≥10 ℃ higher than that of pure nylon film, and the residual carbon content is ≥7%, indicating that it has excellent flame retardancy. The first thermal decomposition temperature shows that the composite film has good thermal stability at 318 ℃, indicating that the service temperature of the composite film can reach 318 ℃. (2) The method of the present invention is simple, efficient, easy to operate, low in cost, and suitable for industrial production. Attached Figure Description

[0045] Figure 1 These are the DSC spectra of the synergistically reinforced flame-retardant polystyrene / nylon composite films obtained in Examples 1-6 of the present invention and the pure nylon film obtained in Comparative Example 7; Figure 2 These are TGA spectra of the synergistically reinforced flame-retardant polystyrene / nylon composite films obtained in Examples 1-6 of this invention and the pure nylon film obtained in Comparative Example 7. Detailed Implementation

[0046] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0047] The polystyrene-based carbonized microspheres used in the embodiments and comparative examples of this invention were all prepared from CN110922787A, Polystyrene-based carbonized microspheres and their preparation method and application example 1. 50 The particle size is 200 nm; the nylon used in the embodiments and comparative examples of this invention is all powder; the raw materials or chemical reagents used in the embodiments and comparative examples of this invention are obtained through conventional commercial means unless otherwise specified.

[0048] For the preparation method of surface carboxylation modified polystyrene microspheres, refer to Examples 1-1 to 1-6. 100 parts by weight of polystyrene-based carbonized microspheres were dried at 100℃ for 6 hours, and then 1, 3, 5, 6, 7, and 9 parts by weight of phenyltetracarboxylic dianhydride were added respectively and mixed. The mixture was then placed in an air jet mill and air jet milled at 80℃ and 0.65MPa for 0.3 hours until D was obtained. 50 Polystyrene microspheres with a particle size of 600 nm were obtained by surface carboxylation modification.

[0049] For the preparation method of surface carboxylation modified polystyrene microspheres, refer to Examples 1-7 to 1-10. 100 parts by weight of polystyrene-based carbonized microspheres were dried at 100℃ for 6 hours, then 6 parts by weight of benzoic acid dianhydride were added and mixed. The mixture was then placed in an air jet mill and air-milled at 80℃ and 0.65MPa for 0.1 hours, 0.2 hours, 0.4 hours, and 0.5 hours, respectively, until D... 50 Polystyrene microspheres with a particle size of 600 nm were obtained by surface carboxylation modification.

[0050] For the preparation method of PA6 nylon formic acid solution, refer to Examples 2-1 to 2-4. The PA6 nylon formic acid solution was obtained by mixing and dissolving PA6 nylon and formic acid solvent at a mass ratio of 1:8 at 45℃, 50℃, 55℃, and 60℃ at a rotation speed of 80 r / min for 2 hours.

[0051] For the preparation method of PA6 nylon organic solution, refer to Examples 2-5 to 2-9. The PA6 nylon organic solution was prepared by mixing and dissolving PA6 nylon with trifluoroacetic acid, m-cresol, 1-butyl-3-methylimidazolium chloride, a phenol / tetrachloroethane mixture (volume ratio 0.3:1), and a calcium chloride / ethanol solution (mass concentration 6%) at a mass ratio of 1:8 at 50 °C and a rotation speed of 80 r / min for 2 h.

[0052] Synergistically reinforced flame-retardant polystyrene / nylon composite films: Examples 1-1 to 1-8 The synergistically enhanced flame-retardant polystyrene / nylon composite film is formed by cross-linking polystyrene microspheres with surface carboxylation modification obtained in Reference Examples 1-4 (1, 2, 3, 4, 5, 6, 7, 8 parts by weight), PA6 nylon formic acid solution obtained in Reference Example 2-2 (98.5 parts by weight), and resorcinol diglycidyl ether acetone solution (6% by weight).

[0053] Preparation methods of synergistically reinforced flame-retardant polystyrene / nylon composite films: Examples 1-1 to 1-8 (1) Interfacial precomposite dispersion: According to the components and weight parts of the synergistically reinforced flame-retardant polystyrene / nylon composite film in Examples 1-1 to 1-8, the PA6 nylon formic acid solution obtained in Reference Example 2-2 was heated and stirred at 55°C and 90 r / min. The polystyrene microspheres with surface carboxylation modification obtained in Reference Example 1-4 were added in three portions at a mass ratio of 5:3:2. The total feeding time was controlled to be 25 min. After the feeding was completed, the temperature and speed were maintained for 4 h to obtain interfacial precomposite dispersions 1-1 to 1-8. (2) Crosslinking reaction: According to the components and weight parts of the synergistically reinforced flame-retardant polystyrene / nylon composite film in Examples 1-1 to 1-8, the interface pre-composite dispersion 1-1 to 1-8 obtained by heating and stirring at 50°C and 95 r / min was slowly added to the resorcinol diglycidyl ether acetone solution at a dropping rate of 30 drops / min. After the addition was completed, the temperature and speed were maintained for 8 hours to obtain the crosslinking composite solution 1-1 to 1-8. (3) Film formation and post-treatment: The cross-linked composite liquids 1-1 to 1-8 obtained in step (2) are cast into the molds and set at 30°C for 2 hours. The pre-set films are then placed in a vacuum drying oven along with the molds and cured at 35°C and -0.08 MPa for 12 hours. After curing, the films are removed and placed in a vacuum drying oven and dried at 55°C and -0.10 MPa for 8 hours. The films are then demolded to obtain synergistically reinforced flame-retardant polystyrene / nylon composite films 1-1 to 1-8.

[0054] Application Examples of Synergistically Reinforced Flame-Retardant Polystyrene / Nylon Composite Films 1-1 to 1-8 Examples 1-1 to 1-8 of the synergistically reinforced flame-retardant polystyrene / nylon composite film were applied to flexible electronic device packaging materials.

[0055] Synergistically reinforced flame-retardant polystyrene / nylon composite films: Examples 2-1 to 2-5 The synergistically enhanced flame-retardant polystyrene / nylon composite film is composed of 6 parts by weight of polystyrene microspheres with surface carboxylation modification obtained in Reference Examples 1-4, 98.5 parts by weight of PA6 nylon formic acid solution obtained in Reference Example 2-2, and 3 parts by weight of resorcinol diglycidyl ether acetone solution (the mass concentrations of Examples 2-1 to 2-5 are 2%, 4%, 7%, 8%, and 10%, respectively).

[0056] Preparation methods of synergistically reinforced flame-retardant polystyrene / nylon composite films: Examples 2-1 to 2-5 The only difference between this embodiment and method embodiments 1-6 is that in step (2), resorcinol diglycidyl ether acetone solution is slowly added at a dropping rate of 30 drops / min according to the components and weight parts of the synergistically reinforced flame-retardant polystyrene / nylon composite film embodiments 2-1 to 2-5. The rest is the same as in method embodiments 1-6.

[0057] Application Examples of Synergistically Reinforced Flame-Retardant Polystyrene / Nylon Composite Films 2-1 to 2-5 Examples 2-1 to 2-5 of the synergistically reinforced flame-retardant polystyrene / nylon composite membranes were applied to high-performance lithium battery separators.

[0058] Synergistically reinforced flame-retardant polystyrene / nylon composite films: Examples 3-1 to 3-5 The synergistically enhanced flame-retardant polystyrene / nylon composite film is composed of 6 parts by weight of polystyrene microspheres with surface carboxylation modification obtained from Reference Examples 1-1 to 1-3, 1-5, and 1-6, 98.5 parts by weight of PA6 nylon formic acid solution obtained from Reference Example 2-2, and 3 parts by weight of resorcinol diglycidyl ether acetone solution.

[0059] Preparation methods of synergistically reinforced flame-retardant polystyrene / nylon composite films: Examples 3-1 to 3-5 The only difference between this embodiment and method embodiments 1-6 is that, in step (1), the surface carboxylated polystyrene microspheres used are those obtained from reference examples 1-1 to 1-3, 1-5, and 1-6. The rest is the same as in method embodiments 1-6.

[0060] Application Examples of Synergistically Reinforced Flame-Retardant Polystyrene / Nylon Composite Films 3-1 to 3-5 Examples 3-1 to 3-5, which feature synergistically reinforced flame-retardant polystyrene / nylon composite films, were applied to flexible circuit board (FPC) substrates.

[0061] Synergistically reinforced flame-retardant polystyrene / nylon composite films: Examples 4-1 to 4-4 The synergistically enhanced flame-retardant polystyrene / nylon composite film is composed of 6 parts by weight of polystyrene microspheres with surface carboxylation modification obtained in Reference Examples 1-7 to 1-10, 98.5 parts by weight of PA6 nylon formic acid solution obtained in Reference Example 2-2, and 3 parts by weight of resorcinol diglycidyl ether acetone solution.

[0062] Preparation methods of synergistically reinforced flame-retardant polystyrene / nylon composite films: Examples 4-1 to 4-4 The only difference between this embodiment and method embodiments 1-6 is that, in step (1), the surface carboxylated polystyrene microspheres used are those obtained from reference examples 1-7 to 1-10. The rest is the same as in method embodiments 1-6.

[0063] Application Examples of Synergistically Reinforced Flame-Retardant Polystyrene / Nylon Composite Films 4-1 to 4-4 Examples 4-1 to 4-4 of the synergistically reinforced flame-retardant polystyrene / nylon composite film were applied to the interior of rail transit vehicles.

[0064] Synergistically reinforced flame-retardant polystyrene / nylon composite films: Examples 5-1 to 5-5 Examples 5-1 to 5-5 are the same as Examples 1-6.

[0065] Preparation methods of synergistically reinforced flame-retardant polystyrene / nylon composite films: Examples 5-1 to 5-4 The only difference between this embodiment and method embodiments 1-6 is that in step (2), the reaction time is maintained at the same temperature and rotation speed for 4 h, 6 h, 9 h, and 10 h respectively, yielding crosslinked composite liquids 5-1 to 5-4. The rest is the same as in method embodiments 1-6.

[0066] Application Examples of Synergistically Reinforced Flame-Retardant Polystyrene / Nylon Composite Films: 5-1 to 5-4 Examples 5-1 to 5-5, which are synergistically reinforced flame-retardant polystyrene / nylon composite films, were applied to special safety protective clothing.

[0067] Synergistically reinforced flame-retardant polystyrene / nylon composite films: Examples 6-1 to 6-3 These embodiments 6-1 to 6-3 are the same as those in embodiments 1-6.

[0068] Preparation methods of synergistically reinforced flame-retardant polystyrene / nylon composite films: Examples 6-1 to 6-3 The only difference between this embodiment and method embodiments 1-6 is that: in step (1), the PA6 nylon formic acid solution used is the same as that obtained in reference examples 2-1, 2-3, and 2-4; and in step (2), the heating and stirring temperatures are 45 ℃, 55 ℃, and 60 ℃, respectively. The rest is the same as method embodiments 1-6.

[0069] Application Examples of Synergistically Reinforced Flame-Retardant Polystyrene / Nylon Composite Films: 6-1 to 6-3 Examples 6-1 to 6-3, which feature synergistically reinforced flame-retardant polystyrene / nylon composite films, were applied to lightweight aerospace interior materials.

[0070] Synergistically reinforced flame-retardant polystyrene / nylon composite films: Examples 7-1 to 7-5 Examples 7-1 to 7-5 are the same as Examples 1-6.

[0071] Preparation methods of synergistically reinforced flame-retardant polystyrene / nylon composite films: Examples 7-1 to 7-5 The only difference between this embodiment and method embodiments 1-6 is that, in step (1), the PA6 nylon formic acid solution obtained in reference example 2-2 is replaced with the PA6 nylon organic solution obtained in reference examples 2-5 to 2-9. The rest is the same as in method embodiments 1-6.

[0072] Application Examples of Synergistically Reinforced Flame-Retardant Polystyrene / Nylon Composite Films 7-1 to 7-5 Examples 7-1 to 7-5, which feature synergistically reinforced flame-retardant polystyrene / nylon composite films, were applied to electrical component encapsulation and insulation materials, respectively.

[0073] Comparative Examples 1-6 The only difference between this comparative example and method examples 1-6 is that in step (1), the surface carboxylated polystyrene microspheres obtained in reference examples 1-4 are replaced with unmodified polystyrene microspheres, calcium carbonate, calcium sulfate, lignin, silica, and talc, respectively, to obtain a nylon composite film. The rest is the same as in method examples 1-6.

[0074] Comparative Example 7 The only difference between this comparative example and Examples 1-6 is that, in step (1), the surface carboxylated polystyrene microspheres obtained in Reference Examples 1-4 are not added, and a pure nylon film is finally obtained. The rest of the method is the same as in Examples 1-6.

[0075] Flame-retardant polystyrene / nylon composite films with synergistic reinforcement obtained in Examples 1-1 to 1-8, Examples 2-1 to 2-5, Examples 3-1 to 3-5, Examples 4-1 to 4-4, Examples 5-1 to 5-4, Examples 6-1 to 6-3, and Examples 7-1 to 7-5, along with nylon composite films obtained in Comparative Examples 1 to 6 and pure nylon films obtained in Comparative Example 7, were demolded and shaped. Tensile strength, elongation at break, and Young's modulus were tested according to the method specified in GB / T 1040.3-2006. Oxygen index was tested according to the method specified in GB / T 2406.2-2009. The first thermal decomposition temperature was tested according to the method specified in GB / T 33047.1 (TGA method) to determine their thermal stability. The test results are shown in Tables 1 to 8.

[0076] Table 1. Test results of the synergistically reinforced flame-retardant polystyrene / nylon composite films obtained in Examples 1-1 to 1-8 of the present invention.

[0077] As shown in Table 1, the tensile strength of Examples 1-1 to 1-8 of the present invention showed a trend of first increasing and then decreasing as the amount of surface carboxylated polystyrene microspheres increased. This is because the polystyrene carbonized microspheres have the functions of reinforcing, toughening and filling polycaprolactam molecules. However, with the increase of surface carboxylated polystyrene microspheres, the gap between polycaprolactam molecules becomes smaller, the intermolecular forces increase, the toughness decreases, and thus the tensile strength decreases.

[0078] As shown in Table 1, in Examples 1-1 to 1-8 of the present invention, the elongation at break decreased with increasing amount of surface carboxylated polystyrene microspheres, while the Young's modulus increased. This is because: Figure 1 As shown, the T of pure nylon film g The temperature was 104.3 °C, while the temperature of Examples 1-6 of the present invention was... gThe temperature of the glass transition temperature rose to 116.2℃ due to the addition of polystyrene microspheres with surface carboxylation modification. This is because the addition of polystyrene microspheres with surface carboxylation modification changed the flexibility between nylon molecules, changed the polarity between molecules, increased the internal rotation, increased the glass transition temperature, and decreased the toughness of the material.

[0079] As shown in Table 1, in Examples 1-1 to 1-8 of the present invention, the oxygen index initially increases with the increase of the amount of surface carboxylated polystyrene microspheres. This is because, with the increase of surface carboxylated polystyrene microspheres, their char promotes the formation of a dense char layer during combustion, causing the oxygen index to rise. The subsequent decrease in the oxygen index is due to the gradual excess of surface carboxylated polystyrene microspheres, resulting in an excessively thick but loose char layer, and the oxygen diffusion coefficient gradually increases to 10 times. Figure 2 As shown, the char residue of the pure nylon film is 0%, while the char residue of the synergistically reinforced flame-retardant polystyrene / nylon composite film obtained in Examples 1-6 of this invention is 7%. The increase in char residue from 0 to 7% indicates that it has a low thermal conductivity and effectively delays heat diffusion; at the same time, the sp of the char is... 2 The hybrid structure can quench combustion free radicals, and TGA data shows that the composite film has good thermal stability before 318 °C.

[0080] Table 2 Test Results of the Synergistically Reinforced Flame-Retardant Polystyrene / Nylon Composite Films in Examples 2-1 to 2-5 of the Present Invention

[0081] As shown in Table 2, the tensile strength, elongation at break, and Young's modulus of Examples 2-1 to 2-5 of the present invention initially increased with the increase of the amount of crosslinking agent. This is because: the crosslinking network was formed; the free radicals generated by the decomposition of the crosslinking agent induced the formation of covalent crosslinking bonds between nylon molecular chains, enhancing the interaction force between molecular chains, improving the rigidity and tensile deformation resistance of the material, and through appropriate crosslinking, the original crystalline structure of nylon (such as α crystal form) could be stabilized, and the crystalline region served as a physical crosslinking point to further strengthen the material. The crosslinking network made the external force more uniformly transmitted and reduced local stress concentration. The subsequent decrease was due to: excessive crosslinking restricting the movement of chain segments, excessively high crosslinking density leading to loss of molecular chain flexibility, brittleness of the material, and easy crack propagation. At the same time, excessive crosslinking agent may destroy the regular structure of nylon, leading to a decrease in crystallinity (crosslinking points hinder the arrangement of crystalline regions), weakening the crystallization strengthening effect; in addition, the decomposition byproducts of the crosslinking agent (such as benzoic acid) may induce hydrolytic degradation of nylon chains, forming microcrack initiation points and causing a decrease in the mechanical properties of the material.

[0082] As shown in Table 2, in Examples 2-1 to 2-5 of the present invention, the oxygen index first increased and then decreased with the increase of the amount of crosslinking agent. The reason for the initial increase in oxygen index is that the crosslinking agent promotes crosslinking, which can form a denser, thermally stable char layer, isolating oxygen and heat. At the same time, the crosslinking network reduces the generation of volatile combustible small molecule fragments during combustion and inhibits gas-phase combustion reaction. The reason for the subsequent decrease in oxygen index is that with the increase of the amount of crosslinking agent, the excessively high crosslinking density makes the char layer too rigid, easy to crack and fall off, and lose its protective function. At the same time, the phenyl free radicals generated by the decomposition of excessive crosslinking agent may participate in the combustion chain reaction, which may promote flame propagation. In addition, excessive crosslinking is accompanied by the breakage of nylon chains, forming a porous structure, which is more conducive to the diffusion of oxygen, thus leading to a decrease in oxygen index. The reason why the first thermal decomposition temperature does not differ significantly is that the first thermal decomposition temperature is mainly determined by the inherent thermal stability of the weakest chemical bond (such as amide bond) in the nylon molecular backbone. The cross-linking reaction mainly changes the connection mode between molecular chains (forming a three-dimensional network), but does not significantly enhance or weaken the strength of the backbone chemical bond itself. Therefore, when the material is heated, its initial decomposition trigger point (i.e., the first thermal decomposition temperature) remains unchanged.

[0083] Table 3 Test results of the synergistically reinforced flame-retardant polystyrene / nylon composite films obtained in Examples 3-1 to 3-5 of the present invention

[0084] As shown in Table 3, the tensile strength, elongation at break, and Young's modulus of Examples 3-1 to 3-5 of the present invention show an initial increase followed by a decrease as the amount of anhydride modifier increases. This is because the influence of the amount of anhydride modifier on mechanical properties stems from the change in interface state and material failure mode. The appropriately increased carboxyl groups and epoxy crosslinking agent form a strong and tough three-dimensional network of "nylon-crosslinking agent-microspheres", achieving efficient stress transfer and enabling the rigid microspheres to fully bear the load, thereby simultaneously improving tensile strength, modulus, and elongation at break. However, excessive modifier leads to over-crosslinking, making the crosslinking network too dense, severely restricting molecular chain movement, and causing matrix embrittlement. At the same time, excessive modification may damage the microsphere bulk structure and generate a severe modulus gradient at the interface, leading to stress concentration and inducing microcracks. Therefore, the relevant properties change from synergistic enhancement to overall deterioration.

[0085] As shown in Table 3, the oxygen index of Examples 3-1 to 3-5 of the present invention showed a trend of first increasing and then decreasing with the increase of the amount of acid anhydride modifier. The reason why the first thermal decomposition temperature did not change significantly is that the oxygen index first increased and then decreased due to the change in the quality of the combustion char layer. Appropriate modification forms a dense and stable three-dimensional cross-linked network, which generates a continuous and tough char layer during combustion, effectively isolating heat and oxygen and improving flame retardancy. However, excessive cross-linking makes the char layer too rigid, easy to crack and peel off, loses its protective function, and leads to a decrease in flame retardant efficiency. The reason why the first thermal decomposition temperature did not change significantly is that this temperature depends on the inherent bond energy of the amide bond in the nylon main chain, which is an intrinsic property of the material. The cross-linking reaction only changes the connection mode between molecular chains and does not strengthen or weaken the main chain chemical bond itself. Therefore, the initial decomposition temperature remains stable.

[0086] Table 4. Test results of the synergistically reinforced flame-retardant polystyrene / nylon composite films obtained in Examples 4-1 to 4-4 of the present invention.

[0087] As shown in Table 4, the tensile strength, elongation at break, and Young's modulus of Examples 4-1 to 4-4 of the present invention show a trend of first increasing and then decreasing with the increase of airflow pulverization time. This is because the airflow pulverization time, by regulating the physical structure and surface chemistry of the microspheres, jointly determines the final properties of the composite material. Within the optimal time range, pulverization reduces the microsphere particle size to the ideal range (e.g., D). 50 =600nm), maximizing the specific surface area, while mechanical activation promotes a full and uniform in-situ solid-phase reaction between the anhydride modifier and the microsphere surface, significantly increasing the surface carboxyl density. This together contributes to a strong "nylon-crosslinking agent-microsphere" three-dimensional crosslinking network, achieving optimal stress transfer, thus synergistically improving mechanical properties; however, excessive pulverization time can lead to excessive breakage of microspheres or even structural damage, causing secondary agglomeration of nanoparticles, which become new stress defect points in the matrix, thus deteriorating the interfacial bonding and leading to a decline in various mechanical properties.

[0088] As shown in Table 4, the oxygen index (flame retardancy) also initially increases and then decreases, while the first thermal decomposition temperature remains stable. This is because the oxygen index depends on the quality of the char layer formed after combustion. The well-formed cross-linked network at the optimal pulverization time effectively binds the molecular chains, promoting the formation of a dense and stable protective char layer, resulting in the highest flame retardant efficiency. Excessive pulverization leads to a decrease in char layer quality (loose and easily detached) due to agglomeration or structural damage, weakening the protective effect. The first thermal decomposition temperature is determined by the inherent bond energy of the amide bonds in the nylon backbone. Cross-linking only changes the connection mode between molecular chains and does not change the strength of this intrinsic chemical bond; therefore, the initial decomposition temperature remains unchanged.

[0089] Table 5. Test results of the synergistically reinforced flame-retardant polystyrene / nylon composite films obtained in Examples 5-1 to 5-4 of the present invention.

[0090] As shown in Table 5, the tensile strength, elongation at break, and Young's modulus of Examples 5-1 to 5-4 of the present invention show an initial increase followed by a decrease with the increase of crosslinking reaction time. This is because: with the increase of reaction time, the crosslinking agent decomposes in the early stage of the reaction to generate free radicals, which triggers covalent crosslinking between nylon molecular chains to form a three-dimensional network structure. At the same time, the crosslinking points restrict the slippage of molecular chains, improve the rigidity and tensile deformation resistance of the material, and increase the tensile strength. With the increase of reaction time, the movement of molecular chains is restricted, and the molecular chain segments are difficult to move effectively. The material loses its toughness and is prone to brittle fracture. At the same time, long-term crosslinking may lead to hydrolysis or oxidative degradation of nylon chains. In particular, acidic byproducts such as benzoic acid produced by the decomposition of the crosslinking agent introduce microcracks or defects, resulting in a decrease in tensile strength, elongation at break, and Young's modulus. In addition, excessively long reaction time may lead to excessive crosslinking in some areas and insufficient crosslinking in other areas, forming stress concentration points.

[0091] As shown in Table 5, the oxygen index of Examples 5-1 to 5-4 of the present invention first increases and then decreases with increasing reaction time. This is because the cross-linked structure promotes char formation during combustion, forming a denser and more stable char layer that isolates oxygen and heat. The cross-linked network reduces the small-molecule combustible gases (such as hydrocarbons and CO) generated during nylon pyrolysis, thus inhibiting gas-phase combustion. However, with increasing reaction time, excessive cross-linking makes the char layer too rigid, prone to cracking or peeling, and losing its protective function. Prolonged reaction may lead to the breakage of the nylon main chain, generating low-molecular-weight fragments, which are more easily volatilized and combusted. The reason why there is no significant difference in the first thermal decomposition temperature is that the first thermal decomposition temperature is mainly determined by the inherent thermal stability of the weakest chemical bond (such as the amide bond) in the nylon molecular main chain.

[0092] Table 6 Test results of the synergistically reinforced flame-retardant polystyrene / nylon composite films obtained in Examples 6-1 to 6-3 of the present invention

[0093] As shown in Table 6, the tensile strength, elongation at break, and Young's modulus of Examples 6-1 to 6-3 of this invention show a trend of first increasing and then decreasing with the increase of the temperature at which the PA6 nylon formic acid solution is generated. This is because when the temperature of the PA6 nylon formic acid solution is moderately increased, the molecular thermal motion is enhanced, promoting the full extension, solvation, and exposure of end groups (-NH2, -COOH) of the nylon chains. This provides an ideal basis for the subsequent construction of a uniform and dense "nylon-crosslinking agent-microsphere" crosslinking network, thereby synergistically improving the mechanical properties. If the temperature is too high, the acid hydrolysis catalysis of the organic formic acid will dominate, breaking the amide bonds of the nylon main chain, leading to a decrease in molecular weight. The low molecular weight segments generated by degradation become inherent defects in the network, severely weakening the material's load-bearing capacity and ductility, thus causing a comprehensive decline in performance.

[0094] As shown in Table 6, the tensile strength, elongation at break, and Young's modulus of Examples 6-1 to 6-3 of the present invention show an initial increase followed by a decrease in the crosslinking reaction temperature. This is because, according to the Arrhenius equation, the decomposition rate constant of the crosslinking agent is k = A·exp(-Ea / RT). For every 10 °C increase in temperature, the free radical generation rate increases by 2 to 3 times, promoting the efficient formation of CC crosslinks between nylon molecular chains. At the same time, the crosslinking network stabilizes the crystalline regions, enabling the material to uniformly transmit stress during stretching, thus increasing the tensile strength. The subsequent decrease is due to the fact that if the temperature is too high, byproducts will be generated, the crystalline structure will be destroyed, and the material properties will decrease.

[0095] As shown in Table 6, the oxygen index and the first thermal decomposition temperature in Examples 6-1 to 6-3 of this invention showed an initial increase followed by a decrease as the temperature of PA6 nylon formic acid solution formation and crosslinking reaction increased. This is because: moderately increasing the temperature optimized the dissolution and crosslinking kinetics, promoting the full extension of nylon molecular chains and exposure of end groups, while accelerating the effective reaction between epoxy groups and nylon end groups and microsphere carboxyl groups, constructing a more complete and dense three-dimensional crosslinking network. High temperature promoted the aromatization of the crosslinking network, increased the char residue, and improved the flame retardant performance. The subsequent decrease is due to: when the temperature is too high, thermally driven chemical degradation becomes dominant. On the one hand, the acidolysis of formic acid and the thermal effect work together, leading to the breakage of amide bonds in the nylon main chain, a decrease in molecular weight, and a weakening of the foundation of the crosslinking network. On the other hand, excessive or too rapid crosslinking reactions may generate local stress and an uneven network. The low-molecular-weight fragments generated by degradation are easily volatilized and combustible, and the damaged network is difficult to form a high-quality char layer, resulting in a decrease in the oxygen index. The breakage of the main chain chemical bonds directly reduces the apparent activation energy required for the initial thermal decomposition of the material, causing a substantial decrease in the first thermal decomposition temperature.

[0096] Table 7 Test results of the synergistically reinforced flame-retardant polystyrene / nylon composite films obtained in Examples 7-1 to 7-5 of the present invention

[0097] As shown in Table 7, when the solvents in the PA6 nylon organic solutions used in Examples 7-1 to 7-5 of this invention are trifluoroacetic acid, m-cresol, 1-butyl-3-methylimidazolium chloride, phenol / tetrachloroethane mixture, and calcium chloride / ethanol solution, respectively, their tensile strength, elongation at break, Young's modulus, oxygen index, and first thermal decomposition temperature do not show significant differences. This indicates that the above solvents can completely replace formic acid, demonstrating the universality of the technical route. However, the above solvents are not chosen arbitrarily: 1) They can all efficiently destroy the amide bond hydrogen bond network of PA6 through unique chemical actions (strong protonation, complexation, ion exchange, etc.), achieving complete molecular-level dissolution and ensuring that the precursor solution has high-quality chemical homogeneity; 2) This shows that the core innovation of this invention, the "interface modification-crosslinking synergy" enhancement mechanism, has excellent chemical stability and adaptability. Its performance does not depend on a specific, expensive, or highly toxic solvent. As long as the two core conditions of "complete dissolution" and "complete removal" are met, the core process of this invention can adapt to and produce high-performance products. The first thermal decomposition temperature is primarily determined by the inherent strength of the chemical bonds in the nylon backbone, an intrinsic property unaffected by the removed temporary solvent.

[0098] Table 8. Comparison of test results of the synergistically reinforced flame-retardant polystyrene / nylon composite films obtained in Comparative Examples 1-6, the nylon composite films obtained in Comparative Examples 1-6, and the pure nylon film obtained in Comparative Example 7.

[0099] As shown in Table 8, the nylon composite films prepared in Examples 1-6 of this invention using surface carboxylated polystyrene microspheres as reinforcing fillers exhibit the best comprehensive mechanical and thermal properties because: Unmodified polystyrene microspheres and the nylon matrix have only weak van der Waals forces, making effective stress transfer difficult. Under external force, the interface will first debond, forming microcracks. These weak interfaces and microcracks become defect centers in the material, rapidly expanding under tension or impact, resulting in a significant decrease in both tensile strength and elongation at break. The reinforcing effects of ordinary calcium carbonate, calcium sulfate, lignin, silica, and talc are not as strong as those of surface carboxylated polystyrene microspheres. In particular, lignin showed numerous spots after film formation during the experiment, indicating that lignin is not well soluble in formic acid. Although the addition of silica as a reinforcing filler provides some advantage in tensile strength, its flame-retardant properties are not ideal.

Claims

1. A synergistically reinforced flame-retardant polystyrene / nylon composite film, characterized in that: It is mainly composed of polystyrene microspheres with surface carboxylation modification, nylon organic solution, and epoxy crosslinking agent organic solution.

2. The synergistically reinforced flame-retardant polystyrene / nylon composite film according to claim 1, characterized in that, The weight parts of each component are as follows: 0.5-15.0 parts of surface-modified polystyrene microspheres, 30-100 parts of nylon organic solution, and 0.1-7.0 parts of epoxy crosslinking agent organic solution; the preparation method of the surface carboxylated polystyrene microspheres is as follows: after drying the polystyrene microspheres, an anhydride modifier is added and mixed, followed by air jet milling to obtain surface carboxylated polystyrene microspheres; the drying temperature is 60-120 °C, and the time is 4-12 h; the amount of the anhydride modifier is equivalent to 1-9% of the mass of the polystyrene microspheres; the D of the polystyrene microspheres is... 50 The particle size is 10–500 nm; the air jet milling temperature is 40–100 °C, the pressure is 0.5–0.8 MPa, and the time is 0.1–0.5 h, until the surface carboxylated modified polystyrene microspheres have a D-value. 50 The particle size is 100–800 nm; the polystyrene-based microspheres include one or more of the following: hollow polystyrene microspheres, carbonized polystyrene-based microspheres, or phosphorus- or nitrogen-containing flame-retardant modified polystyrene microspheres; the anhydride modifiers include one or more of the following: pyromellitic dianhydride, maleic anhydride, succinic anhydride, or phthalic anhydride.

3. The synergistically reinforced flame-retardant polystyrene / nylon composite film according to claim 1 or 2, characterized in that: The nylon organic solution is prepared by mixing and dissolving nylon and an organic solvent at a mass ratio of 1:4 to 12; the mixing and dissolving temperature is 30 to 78 °C, the rotation speed is 50 to 100 r / min, and the time is 0.5 to 4.0 h; the nylon includes one or more of aliphatic nylon, aromatic nylon, copolymer nylon, specially modified nylon, or bio-based nylon; the organic solvent in the nylon organic solution includes one or more of formic acid, trifluoroacetic acid, hexafluoroisopropanol, m-cresol, calcium chloride / ethanol solution, ionic liquid, phenol / tetrachloroethane mixture, dimethyl sulfoxide, or N-methylpyrrolidone; the mass concentration of the epoxy crosslinking agent organic solution is 2 to 10. %; the epoxy crosslinking agent includes one or more of aliphatic glycidyl ethers, aromatic glycidyl ethers, alicyclic or polyamine epoxy compounds; the organic solvent in the organic solution of the epoxy crosslinking agent includes one or more of methanol, ethanol, isopropanol, acetone, butanone, tetrahydrofuran, dimethylformamide or ethyl acetate.

4. A method for preparing a synergistically reinforced flame-retardant polystyrene / nylon composite film as described in any one of claims 1 to 3, characterized in that, Includes the following steps: (1) Interfacial precomposite dispersion: In a heated and stirred nylon organic solution, polystyrene microspheres with carboxylation modification on the surface are added in batches. After the addition is completed, the reaction continues to obtain an interfacial precomposite dispersion. (2) Crosslinking reaction: In the pre-composite dispersion obtained by heating and stirring in step (1), an epoxy crosslinking agent organic solution is slowly added. After the addition is completed, the reaction continues to obtain a crosslinking composite solution. (3) Film formation and post-treatment: The cross-linked composite liquid obtained in step (2) is cast into the mold, left to stand and solidify, and then the pre-shaped film is cured together with the mold, taken out, dried, and demolded to obtain a synergistically reinforced flame-retardant polystyrene / nylon composite film.

5. The method for preparing the synergistically reinforced flame-retardant polystyrene / nylon composite film according to claim 4, characterized in that: In step (1), the heating and stirring temperature is 30-78 ℃ and the rotation speed is 60-150 r / min; the batch addition refers to adding in three batches at a mass ratio of 4-6:2-4:1-3; the total addition time is controlled to be 10-30 min; after the addition is completed, the temperature and rotation speed are maintained for 1-6 h.

6. The method for preparing the synergistically reinforced flame-retardant polystyrene / nylon composite film according to claim 4 or 5, characterized in that: In step (2), the heating and stirring temperature is 30-78 °C and the rotation speed is 80-180 r / min; the slow addition drip rate is 20-40 drops / min; after the addition is completed, the temperature and rotation speed are maintained for 4-12 h.

7. The method for preparing the synergistically reinforced flame-retardant polystyrene / nylon composite film according to any one of claims 4 to 6, characterized in that: In step (3), the static setting temperature is 28–38 °C and the time is 0.4–4.0 h; the curing temperature is 30–40 °C and the time is 2–24 h; the drying temperature is 40–60 °C and the time is 6–12 h; the curing and drying processes are carried out in a vacuum environment or under an inert atmosphere; the vacuum degree of the vacuum environment is -0.08 to -0.10 MPa; the inert atmosphere includes one or more of nitrogen, argon or xenon.

8. An application of the synergistically reinforced flame-retardant polystyrene / nylon composite film as described in any one of claims 1 to 3, characterized in that, The synergistically reinforced flame-retardant polystyrene / nylon composite film as described in any one of claims 1 to 3 can be applied to the fields of electrical component packaging or insulation materials, high-performance lithium battery separators, flexible circuit boards, lightweight interior materials for rail transit or aerospace, or special safety protective clothing.

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