Flame-retardant end-capped modified oligomeric polyamide 6 with controllable molecular weight and method for preparing and using the same

Abstract

This invention relates to a molecular weight-controllable, flame-retardant, end-capped modified oligomeric polyamide 6, its preparation method, and its applications, belonging to the field of polyamide recycling technology. The method involves mixing polyamide 6, a flame retardant, and water, hydrolyzing the polyamide 6 while simultaneously end-capping the hydrolysis products with the flame retardant, and then adjusting the molecular weight distribution of the end-capped hydrolysis products to obtain the final product. The product has a number-average molecular weight of 2000-8000 g / mol, a molecular weight distribution index of 1.3-1.6, a hot water extractable content of 2.0 wt%-3.0 wt%, and a cyclic dimer content of 0.1 wt%-0.5 wt%. The product is then melt-blended with polyamide 6 chips to obtain a flame-retardant polyamide 6 composite material. The preparation method of this invention can control the molecular weight of the oligomeric polyamide 6, reducing the generation of oligomers; the obtained flame-retardant polyamide 6 composite material exhibits excellent performance.

Description

Technical Field

[0001] This invention belongs to the field of polyamide recycling technology, and relates to a molecular weight controllable flame-retardant end-capped modified oligomeric polyamide 6, its preparation method and application. Background Technology

[0002] Polyamide (PA), commonly known as nylon, is one of the five major engineering polymers. Polyamide 6, due to its excellent mechanical properties, abrasion resistance, chemical resistance, and spinnability, is widely used in fibers, engineering plastics, films, electrical and electronic products, and automotive parts. With the continuous growth in the use of polyamide 6 products, the amount of scrap, end-of-life products, and composite material waste containing additives is also increasing, making the demand for resource recycling increasingly prominent.

[0003] In terms of recycling, polyamide 6 can be recycled through two routes: mechanical recycling and chemical recycling. Mechanical recycling is a shorter process, but multiple melting heat histories can lead to a decrease in molecular weight, color deterioration, and accumulation of impurities, making it difficult to meet the requirements of high-end fiber or engineering plastic applications.

[0004] Chemical recycling can achieve a higher level of resource recycling. Typical pathways include depolymerizing and recovering caprolactam or controlling the depolymerization of polyamide 6 into oligomers before repolymerization and reuse. However, existing chemical recycling routes still face several key challenges:

[0005] (1) Energy consumption and separation and purification burden: Depolymerization is usually accompanied by high energy consumption and complex separation and purification processes;

[0006] (2) Molecular weight and end groups are difficult to control stably: chain scission reactions such as hydrolysis / alcohololysis / amineolysis have equilibrium characteristics. If the small molecules (especially water) generated during the reaction cannot be removed in time, they are prone to further chain scission and secondary depolymerization, resulting in the difficulty in locking the target molecular weight window of oligomers, the widening of molecular weight distribution, and poor batch repeatability.

[0007] (3) By-products and extractables: Cyclic oligomers or low molecular weight by-products may be formed during the recycling process, resulting in increased hot water extractables, worse odor and processing stability, and affecting the quality of spinning and products.

[0008] Chinese patent CN117487245A discloses a method for controlling the molecular weight and regenerating waste polyamide 6 (PA6) hydrolysis products. The core idea is to directly prepare low-molecular-weight PA6 with the target molecular weight through a process of "water addition → equilibrium-controlled hydrolysis," instead of completely depolymerizing PA6 into caprolactam monomers. This is followed by further chain growth and melt spinning. While this patent solves the problem of controlling molecular weight, the method requires precise calculation of the amount of water in the gas-liquid equilibrium to control the hydrolysis balance, making the control method complex. Furthermore, this method remains complex and energy-intensive, and essentially still belongs to an equilibrium hydrolysis process under aqueous conditions. The resulting product is usually not a single molecular weight component, but a mixed system containing linear oligomers with different degrees of polymerization, cyclic oligomers, residual monomers, and different end-group segments. Because the chain ends are not directionally stabilized, further chain breakage, cyclization, and chain exchange reactions may still occur during the reaction, making it difficult to completely avoid low-molecular-weight byproducts and hot-water extractables, which affects the stability of subsequent processing and the quality of the finished product.

[0009] US Patent 5169870A discloses a process for the continuous recovery of ε-caprolactam from nylon 6 carpets. The core steps include feeding the carpet into a separator to prepare nylon 6-containing waste, then introducing the waste into a depolymerization reactor, generating a distillate containing ε-caprolactam under a depolymerization catalyst and a high-temperature, high-pressure depolymerization environment, and then purifying it through fractionation, oxidation, concentration, and vacuum distillation to obtain ε-caprolactam that meets the requirements for fiber production. This patent can obtain high-purity caprolactam by completely depolymerizing it into monomers and then distilling it, thus solving the problem (3). However, this method uses a high-temperature, high-pressure depolymerization environment, which consumes a lot of energy. The core of this method is to depolymerize polyamide 6 as completely as possible into ε-caprolactam and then purify and recover it by distillation. Its technical goal is monomer recovery rather than the directional preparation of oligomeric polyamide 6 segments, so it does not solve the problem (2).

[0010] Chinese patent CN116813909A discloses a method for purifying caprolactam from waste polyamide through subcritical hydrolysis at 340-350℃ / 9MPa. However, the hydrolysis of waste polyamide requires high temperature of 340-350℃ and high pressure of 9MPa, which places high demands on the equipment's pressure and temperature resistance. The product purification steps are complicated (such as filtration, extraction, and vacuum distillation), increasing production costs and time consumption. This method uses subcritical water conditions of 340-350℃ and 9MPa. Its technical goal is also to deeply depolymerize and purify polyamide 6 to obtain caprolactam, rather than to stably control the product into oligomeric segments within a specific molecular weight range. Under this high temperature and high pressure hydrolysis environment, the amide bond breaking and product transformation process are violent, the chain length changes continuously, and the end group structure is difficult to stably retain or directionally control. Therefore, this method does not solve the problem of the molecular weight window and end group structure of oligomeric polyamide 6 being difficult to stably control (i.e., problem (2)).

[0011] In flame-retardant applications, polyamide 6 is a flammable polymer, prone to problems such as melting and dripping, flame propagation, and rapid heat release during combustion. This is particularly problematic in applications with high flame-retardant requirements, such as electronics, electrical engineering, rail transportation, automotive, and construction. Flame-retardant modification is often necessary to meet regulatory or standard requirements. Existing polyamide 6 flame-retardant systems primarily employ additive flame retardants (such as phosphorus-containing flame retardants, intumescent flame-retardant systems, and inorganic filler synergistic systems) or reactive flame-retardant monomer / end-group introduction methods.

[0012] Additive systems have simple processes and relatively controllable costs, but during long-term use or repeated hot processing, problems such as flame retardant migration / precipitation, adverse effects on mechanical properties and crystallization behavior, and decreased melt rheology and spinning stability are likely to occur. Reactive systems, although they help improve durability, usually need to be introduced during the polymerization or modification stage, resulting in a longer process chain and higher requirements for reaction window and side reaction control.

[0013] Therefore, it is of great significance to study a molecular weight controllable flame-retardant end-capped modified oligomeric polyamide 6, its preparation method and application, in order to solve the above problems. Summary of the Invention

[0014] The purpose of this invention is to solve the problems existing in the prior art and to provide a molecular weight controllable flame-retardant end-capped modified oligomeric polyamide 6, its preparation method and application.

[0015] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0016] A method for preparing a flame-retardant end-capped modified oligomeric polyamide 6 with controllable molecular weight involves mixing polyamide 6, a flame retardant, and water, then heating the mixture to 200-240°C in a sealed nitrogen or inert gas environment to hydrolyze the polyamide 6 while simultaneously end-capping the hydrolysis products with the flame retardant. The mixture is then cooled to 150-180°C to adjust the molecular weight distribution of the end-capped hydrolysis products, thereby obtaining the flame-retardant end-capped modified oligomeric polyamide 6.

[0017] The flame retardant is a dicarboxylic acid with flame retardant properties;

[0018] The amount of polyamide 6 added, the amount of flame retardant added, and the target molar mass of the flame-retardant end-modified oligomeric polyamide 6 (the target molar mass is numerically the same as the target number-average molecular weight) satisfy the following formula:

[0019] m b =k×m a ×M b / (M c -M b );

[0020] In the formula, m aThe amount of polyamide 6 added is expressed in grams (g); m b The amount of flame retardant added, in grams (g); M c M represents the target molar mass of flame-retardant end-capped modified oligomeric polyamide 6, in g / mol. b The value is the molar mass of the flame retardant, expressed in g / mol; k ranges from 1.1 to 1.5.

[0021] During the high-temperature hydrolysis stage (200~240℃), the reaction system is in a closed, high-temperature, water-containing environment. The kinetic energy of water molecules is significantly increased, and under acidic conditions and high temperature, they participate in the reversible hydrolysis of the amide bonds in the polyamide 6 backbone, causing rapid and random chain breakage and generating a large number of -NH2 and -COOH end groups. This stage is characterized by rapid chain scission; the original polyamide 6 molecular chains with normal chain lengths are quickly broken up, and the main body gradually transforms into oligomeric polyamide 6 segments with active end groups. During this process, the equilibrium shifts towards chain scission, and reversible polycondensation hardly occurs. Simultaneously, the -COOH end groups in the flame retardant molecule preferentially undergo acid-base complexation with the -NH2 end groups of the generated oligomeric segments to form ammonium salts (-NH3). + ··· - The polyamide 6 oligomer (OOC-) undergoes a further end-group amidation reaction at high temperatures, thereby generating flame-retardant-terminated polyamide 6 oligomers. Because this end-group trapping process exhibits preferential close-range association with the newly generated -NH2 end groups, its reaction competitiveness is higher than that of ordinary inter-chain polycondensation, which relies on free diffusion and end-group orientation collisions between oligomers. Therefore, within the process window of this invention, the high-temperature stage exhibits a parallel process of "rapid chain breaking + rapid end-capping," and the end-capping process plays a dominant role in establishing the effective number of chains in the system.

[0022] Specifically, during the high-temperature stage, the newly formed -NH2 end groups are preferentially captured by the -COOH end groups of the flame retardant and converted into flame retardant end-capped segments, thus significantly inhibiting the reaction pathway that could otherwise reduce the number of chains through reversible polycondensation. Therefore, the high-temperature stage not only completes the rapid transformation of polyamide 6 from polymer chains to oligomeric segments, but also simultaneously locks the end groups of most of the active oligomeric segments, resulting in a composition dominated by flame retardant-terminated oligomeric segments. Components not timely end-capped mainly manifest as a small amount of residual active short chains, rather than a large number of original normal-length polyamide 6 molecular chains. Therefore, at the end of the high-temperature stage, the average number of chains and the corresponding number-average molecular weight of the system are predetermined by the effective end-capping amount of the flame retardant; the higher the amount of flame retardant, the more segments are captured and locked, the higher the effective number of chains in the system, and the lower the average molecular weight of the final product; conversely, the lower the amount of flame retardant, the greater the space for reversible polycondensation in the system, and the relatively higher the average molecular weight. Therefore, the final molecular weight can be directionally controlled by the amount of flame retardant added.

[0023] The flame-retardant-terminated polyamide 6 oligomers are stable within this process window and are unlikely to undergo large-scale interchain bridging growth through their residual -COOH end groups. This is because: firstly, under acidic conditions, a large number of unterminated -NH2 end groups in the system are converted into -NH3 end groups. + Firstly, the presence of the flame retardant in the polyamide chain reduces its nucleophilicity, hindering the formation of new amide bonds with the residual -COOH end groups of the flame retardant end-capped segments. Secondly, after one end of the flame retardant is fixed to the polyamide chain end, the adjacent carboxyl group is subject to local steric hindrance, conformational constraints, and diffusion / orientation limitations in the high-viscosity polymer phase, significantly reducing the probability of further bridging to another chain segment. Thirdly, the continuous rapid chain scission and end-capture processes at high temperatures cause the flame retardant to primarily function as an end-capped monofunctional end-capping agent rather than a chain-growing bridging unit. Therefore, statistically, the flame retardant achieves the locking of the effective number of chains in the system at high temperatures, thereby determining the main controlling basis of the final product's molecular weight.

[0024] This method incorporates a low-temperature stage to address the small number of unterminated chain segments remaining after high-temperature chain scission. In the low-temperature equilibrium stage (150-180℃), the reaction system temperature decreases, significantly reducing the vapor pressure of water and the hydrolysis rate, thus weakening further chain scission. At this point, the system is no longer dominated by the original normal-length polyamide 6, but mainly consists of flame-retardant-terminated oligomer segments and a small amount of residual unterminated active short chains. The oligomers generated in the high-temperature stage and the residual water have a certain plasticizing effect on the system, allowing it to retain some chain segment migration ability even below the nominal melting point. Under these conditions, the small number of residual unterminated short chains gradually integrate into the main chain group dominated by flame-retardant-terminated segments through limited amide exchange and limited polycondensation reactions. Specifically, the -NH2 ends of shorter, unterminated oligomer segments undergo limited polycondensation reactions with the terminated flame-retardant oligomers, while the relatively longer residual active segments are redistributed to the medium-length range through limited amide exchange. Therefore, the main role of the low-temperature stage is not to re-determine the final molecular weight, but to further reduce a small number of abnormally short chains, promote the convergence of chain length distribution, and reduce the molecular weight distribution index (PDI) on the basis of not changing the molecular weight main control established by the amount of flame retardant added in the high-temperature stage.

[0025] Therefore, the molecular weight control mechanism of the flame retardant in this invention can be summarized as follows: at high temperature, the effective number of chains controlled by the amount of flame retardant is established through "rapid chain breaking + rapid end-capping", thereby pre-determining the final molecular weight value; at low temperature, the molecular weight distribution is further converged and homogenized through the limited incorporation of a small number of residual uncapped short chains into the capped main chain group and the redistribution of chain segments.

[0026] The choice of an acidic environment is crucial in the above reaction process for the following reasons:

[0027] The amide bond (-CO-NH-) in the polyamide 6 molecular chain itself has high stability. Under acidic conditions, the proton H... + It can react with the carbonyl oxygen in the amide bond, causing partial protonation of the carbonyl group, thereby enhancing the electrophilicity of the carbonyl carbon atom and lowering its activation energy for nucleophilic attack. During the high-temperature hydrolysis stage, this activation, combined with a relatively high heat input, allows water molecules to participate in the amide bond breaking reaction; however, during the low-temperature equilibrium stage, the system's heat energy is insufficient to overcome the activation energy required for complete amide bond breaking, thus significantly inhibiting the chain scission reaction.

[0028] However, under the aforementioned acidic environment, the protonated carbonyl group can still be attacked by the uncapped -NH2 or -COOH end groups at the ends of the polyamide molecular chain, resulting in an amide exchange reaction. This process is a reversible rearrangement between molecular chains, does not involve an increase in the number of molecular chains, and requires a lower activation energy than the complete cleavage reaction of the amide bond. Therefore, it can occur at lower temperatures, thereby achieving the redistribution of molecular chain segments and the regulation of molecular weight distribution.

[0029] To control the molecular weight of the product, this invention controls the amount of polyamide 6 and flame retardant added. The content of the original normal-length polyamide 6 molecular chains and uncapped segments in the product is very low and negligible. Under ideal conditions where neither is present, the amount of polyamide 6 added, the amount of flame retardant added, and the target number-average molecular weight of the flame-retardant-capped modified oligomeric polyamide 6 must satisfy the following formula:

[0030] m b =k×m a ×M b / (M c -M b );

[0031] Among them, M c -M b The target number-average molecular weight (excluding flame retardant) of the oligomeric PA6 main chain segment is M. b / (M c -M b ) represents the molar ratio of flame retardant to polyamide 6, and k is taken as 1.1~1.5 to compensate for insufficient end-capping conversion and maintain an acidic environment;

[0032] In this invention, the target number-average molecular weight of the flame-retardant end-modified oligomeric polyamide 6 obtained by the above formula is used as the number-average molecular weight to be controlled and achieved.

[0033] The specific derivation of this formula is as follows:

[0034] (1) Symbol definition;

[0035] m a : Amount of polyamide 6 added, in grams;

[0036] m b : Amount of flame retardant added, in grams.

[0037] M c Target molar mass of flame-retardant end-capped modified oligomeric polyamide 6, in g / mol;

[0038] M b Molar mass of flame retardant, in g / mol;

[0039] n: Mole of flame-retardant end-capped modified oligomeric polyamide 6, in mol;

[0040] k: Empirical correction coefficient (dimensionless, used to compensate for insufficient end-cap conversion rate, allocation loss, etc.), ranging from 1.1 to 1.5;

[0041] (2) Since the target molar mass of the flame-retardant end-capped modified oligomeric amide 6 is M c It contains one flame retardant molecule with a molar mass of M. b The molar mass of each mole of PA6 main chain segment is:

[0042] ;

[0043] (3) n can be deduced from the molar mass corresponding to the PA6 main chain segment, as follows:

[0044] ;

[0045] (4) Assuming that each flame-retardant end-modified oligomeric polyamide 6 molecular chain corresponds to one flame retardant molecule, then The amount of flame retardant used in the end-capping process is: ;

[0046] (5) Considering insufficient end-capping conversion and maintaining acidic conditions, the actual dosage is obtained by correcting with k:

[0047] ;

[0048] The results were:

[0049] ;

[0050] Therefore, the method of the present invention has simple procedures and the molecular weight of the product is controllable. Furthermore, the oligomer segments with excessively short low-temperature equilibrium stages tend to undergo limited polycondensation and integrate into the main chain due to their high end-group concentration. Combined with the end-capping by flame retardants, cyclization reactions (end-group biting back) and the generation of hot water extractables are avoided as much as possible.

[0051] As a preferred technical solution:

[0052] The preparation method of the molecular weight controllable flame-retardant end-capped modified oligomeric polyamide 6 as described above involves an initial gas pressure of 0.1 MPa in a sealed nitrogen or inert gas environment; heating to 200~240℃ and holding for 2~4 h; cooling to 150~180℃ and holding for 1~3 h.

[0053] The method for preparing a molecular weight controllable flame-retardant end-capped modified oligomeric polyamide 6 as described above, wherein the flame retardant is one or more of the following: phosphorus-containing dicarboxylic acid A, phosphorus-containing dicarboxylic acid B, (6-oxo-6H-dibenzo[c,e][1,2]oxophosphoric acid hexane-6-yl)methyl]succinic acid (DDP), DOPO-maleic acid addition dicarboxylic acid, and DOPO-fumaric acid addition dicarboxylic acid. Specifically, phosphorus-containing dicarboxylic acid A is a phosphorus-containing dicarboxylic acid obtained by the addition of itaconic acid, maleic acid, or fumaric acid with a pH-containing arylphosphinolic acid; and phosphorus-containing dicarboxylic acid B is a phosphorus-containing dicarboxylic acid obtained by the addition of itaconic acid, maleic acid, or fumaric acid with a pH-containing alkylphosphinolic acid.

[0054] Phosphorus-containing dicarboxylic acid A can be prepared by the following method:

[0055] A PH-containing arylphosphonic acid is added to an organic solvent at a molar ratio of 1:(1.00~1.10) with one of itaconic acid dimethyl ester, maleic acid dimethyl ester, or fumarate dimethyl ester. A free radical initiator is added under inert gas protection, and the reaction is carried out at 80~100℃ for 6~12h to obtain a phosphorus-containing dicarboxylic acid ester intermediate. After the reaction is completed, the solvent is removed under reduced pressure, and the obtained intermediate is added to an alcohol / water mixed solvent with an alkaline hydrolysis agent. The reaction is carried out at 40~65℃ for 2~6h for hydrolysis. After hydrolysis, the pH of the system is adjusted to 1~3 with an inorganic acid, and a solid is precipitated. After solid-liquid separation, washing, and drying, the corresponding arylphosphonic acid-itaconic acid type, arylphosphonic acid-maleic acid type, or arylphosphonic acid-fumarate type phosphorus-containing dicarboxylic acid is obtained.

[0056] The organic solvent is preferably one or more of N,N-dimethylformamide, N,N-dimethylacetamide, 1,4-dioxane, toluene, or xylene; the free radical initiator is preferably one or more of azobisisobutyronitrile, azobisisoheptanenitrile, or benzoyl peroxide, and its amount is 1-3 mol% of the amount of arylphosphinolic acid; the alkaline hydrolysis agent is preferably sodium hydroxide or potassium hydroxide; the inorganic acid is preferably hydrochloric acid, sulfuric acid, or phosphoric acid; and the arylphosphinolic acid containing pH is preferably one or more of phenylphosphinolic acid, tolylphosphinolic acid, or naphthylphosphinolic acid.

[0057] Phosphorus-containing dicarboxylic acid B can be prepared by the following method:

[0058] An alkylphosphonic acid containing pH is added to an organic solvent at a molar ratio of 1:(1.00~1.10) with one of itaconic acid dimethyl ester, maleic acid dimethyl ester, or fumarate dimethyl ester. A free radical initiator is added under inert gas protection, and the reaction is carried out at 80~100℃ for 6~12h to obtain a phosphorus-containing dicarboxylic acid ester intermediate. After the reaction is completed, the solvent is removed under reduced pressure, and the obtained intermediate is added to an alcohol / water mixed solvent with an alkaline hydrolysis agent. The reaction is carried out at 40~65℃ for 2~6h for hydrolysis. After hydrolysis, the pH of the system is adjusted to 1~3 with an inorganic acid, and a solid is precipitated. After solid-liquid separation, washing, and drying, the corresponding alkylphosphonic acid-itaconic acid type, alkylphosphonic acid-maleic acid type, or alkylphosphonic acid-fumarate type phosphorus-containing dicarboxylic acid is obtained.

[0059] The organic solvent is preferably one or more of N,N-dimethylformamide, N,N-dimethylacetamide, 1,4-dioxane, toluene, or xylene; the free radical initiator is preferably one or more of azobisisobutyronitrile, azobisisoheptanenitrile, or benzoyl peroxide, and its amount is 1-3 mol% of the alkylphosphine acid; the alkaline hydrolysant is preferably sodium hydroxide or potassium hydroxide; the inorganic acid is preferably hydrochloric acid, sulfuric acid, or phosphoric acid; the alkylphosphine acid containing pH is preferably one or more of methylphosphine acid, ethylphosphine acid, propylphosphine acid, isopropylphosphine acid, or butylphosphine acid.

[0060] The preparation method of flame-retardant end-capped modified oligomeric polyamide 6 with controllable molecular weight as described above involves vacuum drying the polyamide 6 at 90~120℃ for 16~20h before mixing, so that the water content is ≤500ppm.

[0061] The preparation method of a molecular weight controllable flame-retardant end-capped modified oligomeric polyamide 6 as described above, wherein the mass ratio of water to polyamide 6 is 1:1 to 1:10, and the amount of flame retardant added is 5wt% to 17wt% of the amount of polyamide 6 added.

[0062] The present invention also provides a flame-retardant end-capped modified oligomeric amide 6, which is prepared by the method described above for preparing a flame-retardant end-capped modified oligomeric amide 6 with controllable molecular weight; the flame-retardant end-capped modified oligomeric amide 6 has a number average molecular weight of 1000~8000 g / mol, a molecular weight distribution index of 1.3~1.6, a hot water extractable content of 2.0wt%~3.0wt%, and a cyclic dimer content of 0.1wt%~0.5wt%.

[0063] As a preferred technical solution:

[0064] As described in section 6, a flame-retardant end-capped modified oligomeric amide 6 has a relative viscosity of 1.1~1.7 and a melting point of 211.0~216.0℃.

[0065] The present invention also provides a method for preparing a flame-retardant polyamide 6 composite material, wherein a flame-retardant end-modified oligomeric polyamide 6 as described above is melt-blended with polyamide 6 chips to obtain a flame-retardant polyamide 6 composite material.

[0066] The method for preparing a flame-retardant polyamide 6 composite material as described above involves drying the flame-retardant end-capped modified oligomeric polyamide 6 in a vacuum oven at 90-110°C for 16-20 hours before melt blending, so that the water content is ≤500ppm.

[0067] As a preferred technical solution:

[0068] The method for preparing a flame-retardant polyamide 6 composite material as described above involves a mass ratio of flame-retardant end-modified oligomeric polyamide 6 to polyamide 6 chips of 100:20~100; the polyamide 6 chips have a relative viscosity of 2.3~3.2 and a number-average molecular weight of 2.0×10⁻⁶. 4 ~3.5×10 4 g / mol.

[0069] The preparation method of the flame-retardant polyamide 6 composite material as described above includes the following process parameters: the temperature of the twin-screw extruder is 260~280℃, and the screw speed is 100~500rpm.

[0070] The flame-retardant polyamide 6 composite material prepared by the method described above has a tensile strength of 60~85MPa, an elongation at break of 10%~60%, a limiting oxygen index of 28%~32%, a UL-94 V-0 rating, and no ignition dripping.

[0071] Beneficial effects:

[0072] (1) This invention does not aim at complete depolymerization to monomers, but rather controls the degradation of waste PA6 into flame-retardant end-capped oligomeric PA6 with a target molecular weight window and designable end groups. Compared with the complete depolymerization-purification-repolymerization route, this application avoids the high dependence on monomer-level purification, reduces the energy-intensive complete depolymerization step and the complex liquid-phase separation and purification process, while retaining some of the polymerization value of the original polyamide segments. Therefore, it has advantages in terms of energy consumption, process length and adaptability to complex waste materials.

[0073] (2) This invention does not control molecular weight solely by adjusting hydrolysis equilibrium, but introduces a flame-retardant dicarboxylic acid during chain scission to cap and fix the active chain ends, thereby statistically locking the number of chains and the end-group structure. This not only allows for stable product control within the target molecular weight window but also reduces the risk of continued chain scission and secondary depolymerization in the later stages of the reaction and post-treatment, thus improving molecular weight distribution and end-group stability. Unlike CN117487245A, which primarily relies on water addition to adjust hydrolysis equilibrium to control the degree of polymerization, this application uses a "chain scission-end-capping coupling" method to chemically fix the active chain ends while controlling the molecular weight. Therefore, the controlled objects of this application include not only the target number-average molecular weight but also the number of chains and the end-group structure itself. Furthermore, this application simultaneously introduces flame-retardant end groups, achieving synergy between recycling and functional modification, resulting in higher added value than regeneration routes that only control molecular weight.

[0074] (3) In the preparation method of a molecular weight controllable flame-retardant end-capped modified oligomeric polyamide 6 of the present invention, the hydrolyzed polyamide 6 segments are end-capped by flame retardant, thereby avoiding the occurrence of cyclization reaction of polyamide 6 segments (cyclization reaction caused by end-group biting back of polyamide 6 segments), and by controlling the molecular weight distribution in the low temperature equilibrium stage, the excessively short oligomeric segments are incorporated into the main chain of the system through condensation polymerization, thereby avoiding the occurrence of low molecular weight by-products;

[0075] (4) The flame-retardant end-capped modified oligomeric polyamide 6 of the present invention has a controllable molecular weight, a narrow molecular weight distribution index, and low content of hot water extractables and cyclic dimers.

[0076] (5) The preparation method of the flame-retardant polyamide 6 composite material of the present invention is simple and the obtained flame-retardant polyamide 6 composite material has excellent performance. Attached Figure Description

[0077] Figure 1 NMR of flame-retardant end-capped modified oligomeric polyamide 6 of Example 1 1 H spectrum;

[0078] Figure 2 The equation for the hydrolysis and capping reaction of DDP, water, and polyamide 6 is given. Detailed Implementation

[0079] The present invention will be further described below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0080] The testing method in this embodiment is as follows:

[0081] Number-average molecular weight and molecular weight distribution (PDI): The number-average molecular weight and molecular weight distribution (PDI) of the samples were tested using a GPC-50 gel permeation chromatography system from PL Corporation, UK. This system is equipped with a differential refractive index detector and a PL gel column (5 μm mixed C). 1,1,1,3,3,3-hexafluoro-2-propanol was used as the eluent, and the flow rate was 1 mL / min. Before testing, the samples were dried and dissolved in hexafluoroisopropanol to prepare a solution with a concentration of 1.0 mg / mL. The test was performed when the column temperature reached 40 ± 1 °C.

[0082] Relative viscosity: Refer to GB / T 38138-2019 Test method for fiber grade polycaprolactam (PA6) slices, section 5.2, relative viscosity.

[0083] Melting point: The crystallization and melting behavior of the sample was tested using a TA-Q20 differential scanning calorimeter. A dried sample of 5.0 ± 0.2 mg was placed in an aluminum crucible and tested under a nitrogen atmosphere at a flow rate of 50 mL / min. The program was as follows: heating from 30 °C to 260 °C at a rate of 10 °C / min, holding for 3 min; then cooling to 30 °C at a rate of 10 °C / min; and finally heating back to 260 °C at a rate of 10 °C / min. The melting point was determined by the peak temperature of the endothermic melting peak in the second heating curve.

[0084] Hot water extractable content: The hot water extractable content was tested according to the gravimetric method in 5.3.1 of GB / T 38138-2019.

[0085] Cyclic dimer content: Different types of cyclic oligomers in the samples were qualitatively and quantitatively analyzed using a Shimadzu LC-16 high-performance liquid chromatograph (HPLC) equipped with a WondaSil C18-WR (200 mm, 5 μm packed particle size) column and a UV detector. The detection wavelength was 210 nm, and the detection temperature was 40°C. A binary gradient assay was used, with methanol and water as the mobile phases. The assay method is shown in the table below.

[0086] Testing phase Time (min) Methanol percentage (Vol%) 1 2 10 2 17 70 3 18 70 4 18.1 10 5 20 10

[0087] Tensile strength: determined according to GB / T 1040.2-2022. The specimens are made of type 1A specimens and injection molded according to GB / T 17037.1-2019. The specimens are conditioned for 40 hours at 23±2℃ and 50±10% relative humidity before testing. The tensile speed is 50 mm / min. Five specimens are tested in parallel, and the arithmetic mean is taken as the result.

[0088] Elongation at break: Determined according to GB / T 1040.2-2022, using type 1A specimens, injection molded according to GB / T 17037.1-2019, the specimens were conditioned for 40h at 23±2℃ and 50% relative humidity, the tensile speed was 50mm / min, 5 specimens were tested in parallel, and the arithmetic mean was taken.

[0089] Limiting Oxygen Index (LOI): The sample size was 80 mm × 10 mm × 4 mm, and the test was conducted using the top-ignition method. Before testing, the sample was conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for at least 88 hours. During testing, the sample was vertically fixed inside the combustion chamber, and a nitrogen / oxygen mixture was introduced into the combustion chamber from bottom to top, with the apparent gas flow rate controlled at 40 mm / s. The initial oxygen concentration was set at 25.0%, and then the oxygen concentration was gradually adjusted in increments of 0.2%. The top of the sample was ignited under each oxygen concentration condition, and the minimum oxygen concentration required for the sample to maintain the specified combustion was determined. The limiting oxygen index is expressed as an oxygen volume fraction, and the result is retained to one decimal place.

[0090] Flame retardancy rating: Tested according to UL94 standard using the vertical burning method. The sample size is 125mm × 13mm × 1.6mm, with 5 samples tested per group. After vertically fixing the sample, a flame is applied to the lower end of the sample twice, each time for 10 seconds. The flaming time after the first application and the afterglow time after the second application are recorded. It is also observed whether the sample burns to the fixture and whether the dripping material ignites the cotton below. The flame retardancy rating of the material is ultimately determined as V-0, V-1, or V-2 based on the test results.

[0091] The preparation method of the phenylphosphinoic acid-itaconic acid addition dicarboxylic acid in the following examples is as follows:

[0092] (1) Add 7.10 g phenylphosphine (0.050 mol), 8.07 g dimethyl itaconic acid (0.051 mol) and 50 mL N,N-dimethylformamide (DMF) to a 250 mL four-necked flask equipped with a stirrer, thermometer, reflux condenser and nitrogen inlet device, and stir and mix evenly at room temperature; then purge with nitrogen for 20 min to remove air from the reaction system;

[0093] (2) Add 0.164 g of azobisisobutyronitrile (0.0010 mol) to the system in step (1), then heat to 88 °C under nitrogen protection and react at this temperature for 9 h; after the reaction is completed, cool to room temperature and remove the solvent DMF by vacuum distillation to obtain the crude product;

[0094] (3) Add the crude product from step (2) to a mixed solvent consisting of 80 mL methanol and 20 mL deionized water, stir and disperse evenly, then add 4.40 g sodium hydroxide (0.110 mol) to the system, heat to 55 °C, and react at this temperature for 4 h to hydrolyze the ester group.

[0095] (4) Cool the system of step (3) to room temperature, adjust the pH of the system to 2 with 2 mol / L hydrochloric acid, and precipitate a white to off-white solid; after separation by vacuum filtration, the obtained filter cake is washed with deionized water until the filtrate is close to neutral, and then vacuum dried at 60°C for 12 h to obtain the target product, namely phenylphosphine-itaconic acid addition dicarboxylic acid.

[0096] The preparation steps of the methylphosphinoic acid-itaconic acid type phosphorus-containing dicarboxylic acid in the following examples are as follows:

[0097] (1) 9.6 g of methylphosphonic acid, 15.8 g of dimethyl itaconic acid and 80 mL of N,N-dimethylformamide were added to a four-necked flask equipped with a stirrer, thermometer and reflux condenser. After stirring and dissolving at room temperature, nitrogen gas was purged for 20 min to remove oxygen from the system. Then 0.35 g of azobisisobutyronitrile was added, and the temperature was raised to 85 °C. The reaction was carried out under nitrogen protection for 8 h. After the reaction was completed, the temperature was lowered to room temperature and the solvent was removed under reduced pressure to obtain the crude product of phosphorus-containing dicarboxylic acid ester intermediate.

[0098] (2) Add the crude product of phosphorus-containing dicarboxylic acid ester intermediate from step (1) to 100 mL of a mixed solution of methanol and water (volume ratio of methanol to water 3:1), add 20% NaOH aqueous solution by mass while stirring, control the temperature at 50℃, and react for 4 h to carry out hydrolysis.

[0099] (3) Adjust the pH of the reaction system in step (2) to 2 with 2 mol / L hydrochloric acid to precipitate solid. Then filter, wash with deionized water until the filtrate is nearly neutral, and dry under vacuum at 60°C for 12 h to obtain methylphosphinoic acid-itaconic acid type phosphorus-containing dicarboxylic acid.

[0100] Both DOPO-maleic acid addition dicarboxylic acid and DOPO-fumaric acid addition dicarboxylic acid were prepared according to Example 2 in CN103965245B.

[0101] Example 1

[0102] A method for preparing flame-retardant end-capped modified oligomeric amide 6 with controllable molecular weight, wherein the target number-average molecular weight of the flame-retardant end-capped modified oligomeric amide 6 is 2380 g / mol, and the specific steps are as follows:

[0103] (1) Preparation of raw materials;

[0104] Polyamide 6: Polyamide 6 after vacuum drying at 90°C for 20 hours; the manufacturer of polyamide 6 is BASF, and the brand name is Ultramid B3S;

[0105] Flame retardant: DDP;

[0106] water;

[0107] (2) such as Figure 2 As shown, polyamide 6, flame retardant and water were mixed and heated to 240°C in a sealed nitrogen environment (0.1 MPa) for 4 hours, and then cooled to 150°C and held for 1 hour to obtain flame-retardant end-modified oligomeric polyamide 6.

[0108] The mass ratio of water to polyamide 6 is 1:1; the amount of polyamide 6 added is 100g; and the amount of flame retardant added is 17wt% of the amount of polyamide 6 added.

[0109] The amount of polyamide 6 added, the amount of flame retardant added, and the target molar mass of the flame-retardant end-capped modified oligomeric polyamide 6 satisfy the following formula:

[0110] m b =k×m a ×M b / (M c -M b );

[0111] In the formula, m a The amount of polyamide 6 added is expressed in grams (g); m b The amount of flame retardant added, in grams (g); M c M represents the target molar mass of flame-retardant end-capped modified oligomeric polyamide 6, in g / mol. b is the molar mass of the flame retardant, in g / mol; k is 1.

[0112] like Figure 1 The image shows the 1H NMR spectrum of the prepared flame-retardant end-capped modified oligomeric amide 6. The number-average molecular weight of the prepared flame-retardant end-capped modified oligomeric amide 6 is 2000 g / mol, the molecular weight distribution index is 1.6, the relative viscosity is 1.1, the melting point is 211℃, the hot water extractable content is 3 wt%, and the cyclic dimer content is 0.5 wt%. The actual measured value of the number-average molecular weight of the prepared flame-retardant end-capped modified oligomeric amide 6 deviates from the theoretical value in the formula, which is mainly related to measurement error and the end-capping conversion rate of the flame retardant.

[0113] Example 2

[0114] A method for preparing flame-retardant end-capped modified oligomeric amide 6 with controllable molecular weight, wherein the target number-average molecular weight of the flame-retardant end-capped modified oligomeric amide 6 is 4072 g / mol, and the specific steps are as follows:

[0115] (1) Preparation of raw materials;

[0116] Polyamide 6: Polyamide 6 dried under vacuum at 95°C for 19.5 h; the manufacturer of polyamide 6 is BASF, and the brand name is Ultramid B3S;

[0117] Flame retardant: DDP;

[0118] water;

[0119] (2) After mixing polyamide 6, flame retardant and water, the mixture is heated to 235°C in a sealed nitrogen environment (0.1MPa) and kept at that temperature for 3.5h, and then cooled to 155°C and kept at that temperature for 1.5h, thereby obtaining flame-retardant end-modified oligomeric polyamide 6.

[0120] The mass ratio of water to polyamide 6 is 1:2; the amount of polyamide 6 added is 100g; and the amount of flame retardant added is 13wt% of the amount of polyamide 6 added.

[0121] The amount of polyamide 6 added, the amount of flame retardant added, and the target molar mass of the flame-retardant end-capped modified oligomeric polyamide 6 satisfy the following formula:

[0122] m b =k×m a ×M b / (M c -M b );

[0123] In the formula, m a The amount of polyamide 6 added is expressed in grams (g); m b The amount of flame retardant added, in grams (g); M c M represents the target molar mass of flame-retardant end-capped modified oligomeric polyamide 6, in g / mol. b The value is the molar mass of the flame retardant, expressed in g / mol; k is 1.4.

[0124] The flame-retardant end-capped modified oligomeric polyamide 6 prepared had a number-average molecular weight of 4200 g / mol, a molecular weight distribution index of 1.52, a relative viscosity of 1.2, a melting point of 212℃, a hot water extractable content of 2.8 wt%, and a cyclic dimer content of 0.4 wt%.

[0125] Example 3

[0126] A method for preparing flame-retardant end-capped modified oligomeric amide 6 with controllable molecular weight, wherein the target number-average molecular weight of the flame-retardant end-capped modified oligomeric amide 6 is 4844 g / mol, and the specific steps are as follows:

[0127] (1) Preparation of raw materials;

[0128] Polyamide 6: Polyamide 6 after vacuum drying at 100°C for 19 hours; the manufacturer of polyamide 6 is BASF, and the brand name is Ultramid B3S;

[0129] Flame retardant: DDP;

[0130] water;

[0131] (2) After mixing polyamide 6, flame retardant and water, the mixture is heated to 230°C in a sealed nitrogen environment (0.1MPa) and kept at that temperature for 3 hours, and then cooled to 160°C and kept at that temperature for 2 hours, thereby obtaining flame-retardant end-modified oligomeric polyamide 6.

[0132] The mass ratio of water to polyamide 6 is 1:4; the amount of polyamide 6 added is 100g; and the amount of flame retardant added is 10wt% of the amount of polyamide 6 added.

[0133] The amount of polyamide 6 added, the amount of flame retardant added, and the target molar mass of the flame-retardant end-capped modified oligomeric polyamide 6 satisfy the following formula:

[0134] m b =k×m a ×M b / (M c -M b );

[0135] In the formula, m a The amount of polyamide 6 added is expressed in grams (g); m b The amount of flame retardant added, in grams (g); M c M represents the target molar mass of flame-retardant end-capped modified oligomeric polyamide 6, in g / mol. b The value represents the molar mass of the flame retardant, expressed in g / mol; k is 1.3.

[0136] The flame-retardant end-capped modified oligomeric polyamide 6 prepared had a number-average molecular weight of 4600 g / mol, a molecular weight distribution index of 1.45, a relative viscosity of 1.32, a melting point of 213℃, a hot water extractable content of 2.6 wt%, and a cyclic dimer content of 0.3 wt%.

[0137] Example 4

[0138] A method for preparing flame-retardant end-capped modified oligomeric amide 6 with controllable molecular weight, wherein the target number-average molecular weight of the flame-retardant end-capped modified oligomeric amide 6 is 4150 g / mol, and the specific steps are as follows:

[0139] (1) Preparation of raw materials;

[0140] Polyamide 6: Polyamide 6 after vacuum drying at 105°C for 18 hours; the manufacturer of polyamide 6 is BASF, and the brand name is Ultramid B3S;

[0141] Flame retardant: Methylphosphonic acid-itaconic acid type phosphorus-containing dicarboxylic acid (structural formula: (Number average molecular weight is 210.10 g / mol).

[0142] water;

[0143] (2) After mixing polyamide 6, flame retardant and water, the mixture is heated to 220°C in a sealed nitrogen environment (0.1MPa) and kept at that temperature for 2.5h, and then cooled to 170°C and kept at that temperature for 2.5h, thereby obtaining flame-retardant end-modified oligomeric polyamide 6.

[0144] The mass ratio of water to polyamide 6 is 1:6; the amount of polyamide 6 added is 100g; and the amount of flame retardant added is 8wt% of the amount of polyamide 6 added.

[0145] The amount of polyamide 6 added, the amount of flame retardant added, and the target molar mass of the flame-retardant end-capped modified oligomeric polyamide 6 satisfy the following formula:

[0146] m b =k×m a ×M b / (M c -M b );

[0147] In the formula, m a The amount of polyamide 6 added is expressed in grams (g); m b The amount of flame retardant added, in grams (g); M c M represents the target molar mass of flame-retardant end-capped modified oligomeric polyamide 6, in g / mol. b The value is the molar mass of the flame retardant, expressed in g / mol; k is 1.5.

[0148] The flame-retardant end-capped modified oligomeric polyamide 6 obtained has a number average molecular weight of 4500 g / mol, a molecular weight distribution index of 1.4, a relative viscosity of 1.45, a melting point of 214℃, a hot water extractable content of 2.4 wt%, and a cyclic dimer content of 0.25 wt%.

[0149] Example 5

[0150] A method for preparing flame-retardant end-capped modified oligomeric amide 6 with controllable molecular weight, wherein the target number-average molecular weight of the flame-retardant end-capped modified oligomeric amide 6 is 5942 g / mol, and the specific steps are as follows:

[0151] (1) Preparation of raw materials;

[0152] Polyamide 6: Polyamide 6 after vacuum drying at 110°C for 17 hours; the manufacturer of polyamide 6 is BASF, and the brand name is Ultramid B3S;

[0153] Flame retardant: Phenylephrine-itaconic acid addition dicarboxylic acid (structural formula: (Number average molecular weight is 272.19 g / mol)

[0154] water;

[0155] (2) After mixing polyamide 6, flame retardant and water, the mixture is heated to 210°C in a sealed nitrogen environment (0.1MPa) and kept at that temperature for 2 hours, and then cooled to 175°C and kept at that temperature for 3 hours, thereby obtaining flame-retardant end-modified oligomeric polyamide 6.

[0156] The mass ratio of water to polyamide 6 is 1:8; the amount of polyamide 6 added is 100g; and the amount of flame retardant added is 6wt% of the amount of polyamide 6 added.

[0157] The amount of polyamide 6 added, the amount of flame retardant added, and the target molar mass of the flame-retardant end-capped modified oligomeric polyamide 6 satisfy the following formula:

[0158] m b =k×m a ×M b / (M c -M b );

[0159] In the formula, m a The amount of polyamide 6 added is expressed in grams (g); m b The amount of flame retardant added, in grams (g); M c M represents the target molar mass of flame-retardant end-capped modified oligomeric polyamide 6, in g / mol. b The value represents the molar mass of the flame retardant, expressed in g / mol; k is 1.25.

[0160] The flame-retardant end-capped modified oligomeric polyamide 6 obtained has a number-average molecular weight of 6200 g / mol, a molecular weight distribution index of 1.35, a relative viscosity of 1.58, a melting point of 215℃, a hot water extractable content of 2.2 wt%, and a cyclic dimer content of 0.2 wt%.

[0161] Example 6

[0162] A method for preparing flame-retardant end-capped modified oligomeric amide 6 with controllable molecular weight, wherein the target number-average molecular weight of the flame-retardant end-capped modified oligomeric amide 6 is 7973 g / mol, and the specific steps are as follows:

[0163] (1) Preparation of raw materials;

[0164] Polyamide 6: Polyamide 6 after vacuum drying at 120°C for 16 hours; the manufacturer of polyamide 6 is BASF, and the brand name is Ultramid B3S;

[0165] Flame retardant: a mixture of DOPO-maleic acid addition dicarboxylic acid and DOPO-fumaric acid addition dicarboxylic acid in a mass ratio of 1:1;

[0166] water;

[0167] (2) After mixing polyamide 6, flame retardant and water, the mixture is heated to 200°C in a sealed nitrogen environment (0.1MPa) and kept at that temperature for 2 hours, and then cooled to 180°C and kept at that temperature for 3 hours, thereby obtaining flame-retardant end-modified oligomeric polyamide 6.

[0168] The mass ratio of water to polyamide 6 is 1:10; the amount of polyamide 6 added is 100g; and the amount of flame retardant added is 5wt% of the amount of polyamide 6 added.

[0169] The amount of polyamide 6 added, the amount of flame retardant added, and the target molar mass of the flame-retardant end-capped modified oligomeric polyamide 6 satisfy the following formula:

[0170] m b =k×m a ×M b / (M c -M b );

[0171] In the formula, m a The amount of polyamide 6 added is expressed in grams (g); m b The amount of flame retardant added, in grams (g); M c M represents the target molar mass of flame-retardant end-capped modified oligomeric polyamide 6, in g / mol. b The value represents the molar mass of the flame retardant, expressed in g / mol; k is 1.15.

[0172] When the flame retardant is a mixture of two or more components, M in the formula b Let M represent the equivalent average molar mass of the mixed flame retardant; let M be the molar mass of the i-th component in the mixed flame retardant. i The mass fraction is ω i The equivalent average molar mass of the mixed flame retardant is calculated using the following formula:

[0173]

[0174] In this embodiment, the flame retardant is a mixture of DOPO-maleic acid addition dicarboxylic acid and DOPO-fumaric acid addition dicarboxylic acid in a mass ratio of 1:1; since the molar mass of the two is the same, M in the formula is... bThe common molar mass of the two is taken as 332.24 g / mol.

[0175] The flame-retardant end-capped modified oligomeric polyamide 6 prepared had a number-average molecular weight of 8000 g / mol, a molecular weight distribution index of 1.3, a relative viscosity of 1.7, a melting point of 216℃, a hot water extractable content of 2 wt%, and a cyclic dimer content of 0.1 wt%.

[0176] Example 7

[0177] A method for preparing a flame-retardant polyamide 6 composite material involves drying a flame-retardant end-modified oligomeric polyamide 6 obtained in Example 1 in a vacuum oven at 90°C for 20 hours, followed by melt blending with polyamide 6 chips to obtain the flame-retardant polyamide 6 composite material. The polyamide 6 chips have a relative viscosity of 3.2 and a number-average molecular weight of 3.5 × 10⁻⁶. 4 g / mol; The mass ratio of flame-retardant end-capped modified oligomeric polyamide 6 to polyamide 6 chips is 100:100;

[0178] The process parameters include: the twin-screw extruder has 7 temperature zones, with temperatures of 260℃, 265℃, 270℃, 275℃, 278℃, 280℃, and 278℃ respectively from zone 1 to zone 7, and a screw speed of 100 rpm;

[0179] The obtained flame-retardant polyamide 6 composite material has a tensile strength of 60 MPa, an elongation at break of 10%, a limiting oxygen index of 32%, a UL-94 V-0 rating, and no ignition dripping.

[0180] Example 8

[0181] A method for preparing a flame-retardant polyamide 6 composite material involves drying a flame-retardant end-modified oligomeric polyamide 6 obtained in Example 2 in a vacuum oven at 95°C for 19.5 hours, followed by melt blending with polyamide 6 chips to obtain the flame-retardant polyamide 6 composite material. The polyamide 6 chips have a relative viscosity of 3 and a number-average molecular weight of 3.2 × 10⁻⁶. 4 g / mol; The mass ratio of flame-retardant end-capped modified oligomeric polyamide 6 to polyamide 6 chips is 100:80;

[0182] The process parameters include: the twin-screw extruder has 7 temperature zones, with temperatures of 260℃, 264℃, 268℃, 272℃, 275℃, 278℃ and 276℃ for zones 1 to 7 respectively, and a screw speed of 180 rpm;

[0183] The obtained flame-retardant polyamide 6 composite material has a tensile strength of 66 MPa, an elongation at break of 18%, a limiting oxygen index of 31.5%, a UL-94 V-0 rating, and no ignition dripping.

[0184] Example 9

[0185] A method for preparing a flame-retardant polyamide 6 composite material involves drying a flame-retardant end-modified oligomeric polyamide 6 obtained in Example 3 in a vacuum oven at 100°C for 19 hours, followed by melt blending with polyamide 6 chips to obtain the flame-retardant polyamide 6 composite material. The polyamide 6 chips have a relative viscosity of 2.8 and a number-average molecular weight of 2.9 × 10⁻⁶. 4 g / mol; The mass ratio of flame-retardant end-capped modified oligomeric polyamide 6 to polyamide 6 chips is 100:60;

[0186] The process parameters include: the twin-screw extruder has 7 temperature zones, with temperatures of 260℃, 263℃, 266℃, 270℃, 273℃, 276℃ and 274℃ for zones 1 to 7 respectively, and a screw speed of 250 rpm;

[0187] The obtained flame-retardant polyamide 6 composite material has a tensile strength of 74 MPa, an elongation at break of 30%, a limiting oxygen index of 31%, a UL-94 V-0 rating, and no ignition dripping.

[0188] Example 10

[0189] A method for preparing a flame-retardant polyamide 6 composite material involves drying a flame-retardant end-modified oligomeric polyamide 6 obtained in Example 4 in a vacuum oven at 105°C for 18 hours, followed by melt blending with polyamide 6 chips to obtain the flame-retardant polyamide 6 composite material. The polyamide 6 chips have a relative viscosity of 2.6 and a number-average molecular weight of 2.6 × 10⁻⁶. 4 g / mol; The mass ratio of flame-retardant end-capped modified oligomeric polyamide 6 to polyamide 6 chips is 100:40;

[0190] The process parameters include: the twin-screw extruder has 7 temperature zones, with temperatures of 260℃, 262℃, 265℃, 268℃, 271℃, 274℃ and 272℃ for zones 1 to 7 respectively, and a screw speed of 320 rpm;

[0191] The obtained flame-retardant polyamide 6 composite material has a tensile strength of 80 MPa, an elongation at break of 45%, a limiting oxygen index of 30%, a UL-94 V-0 rating, and no ignition dripping.

[0192] Example 11

[0193] A method for preparing a flame-retardant polyamide 6 composite material involves drying a flame-retardant end-capped modified oligomeric polyamide 6 obtained in Example 5 in a vacuum oven at 110°C for 17 hours, followed by melt blending with polyamide 6 chips to obtain the flame-retardant polyamide 6 composite material. The polyamide 6 chips have a relative viscosity of 2.4 and a number-average molecular weight of 2.3 × 10⁻⁶. 4g / mol; The mass ratio of flame-retardant end-capped modified oligomeric polyamide 6 to polyamide 6 chips is 100:30;

[0194] The process parameters include: the twin-screw extruder has 7 temperature zones, with temperatures of 260℃, 261℃, 264℃, 267℃, 270℃, 273℃ and 271℃ for zones 1 to 7 respectively, and a screw speed of 400 rpm;

[0195] The obtained flame-retardant polyamide 6 composite material has a tensile strength of 83 MPa, an elongation at break of 55%, a limiting oxygen index of 29%, a UL-94 V-0 rating, and no ignition dripping.

[0196] Example 12

[0197] A method for preparing a flame-retardant polyamide 6 composite material involves drying a flame-retardant end-capped modified oligomeric polyamide 6 obtained in Example 6 in a vacuum oven at 110°C for 16 hours, followed by melt blending with polyamide 6 chips to obtain the flame-retardant polyamide 6 composite material. The polyamide 6 chips have a relative viscosity of 2.3 and a number-average molecular weight of 2 × 10⁻⁶. 4 g / mol; The mass ratio of flame-retardant end-capped modified oligomeric polyamide 6 to polyamide 6 chips is 100:20;

[0198] The process parameters include: the twin-screw extruder has 7 temperature zones, with temperatures of 260℃, 260℃, 263℃, 266℃, 269℃, 272℃ and 270℃ for zones 1 to 7 respectively, and a screw speed of 500 rpm;

[0199] The obtained flame-retardant polyamide 6 composite material has a tensile strength of 85 MPa, an elongation at break of 60%, a limiting oxygen index of 28%, a UL-94 V-0 rating, and no ignition dripping.

[0200] In the preparation process of the molecular weight controllable flame-retardant end-capped modified oligomeric polyamide 6 of the present invention, due to the sealed nitrogen environment, high pressure is generated as the temperature rises. This high pressure is caused by the evaporation of water and has little impact on the experimental results.

Claims

1. A method for preparing a molecular weight controllable flame-retardant end-capped modified oligomeric polyamide 6, characterized in that, Polyamide 6, flame retardant and water are mixed and heated to 200~240℃ in a sealed nitrogen or inert gas environment to hydrolyze polyamide 6. At the same time, the hydrolysis products are end-capped by flame retardant. Then the temperature is lowered to 150~180℃ to adjust the molecular weight distribution of the end-capped hydrolysis products, thereby obtaining flame-retardant end-capped modified oligomeric polyamide 6. The flame retardant is a dicarboxylic acid with flame retardant properties; The amount of polyamide 6 added, the amount of flame retardant added, and the target molar mass of the flame-retardant end-capped modified oligomeric polyamide 6 satisfy the following formula: m b = k x m a x M b / (M c - M b ); wherein m a is the addition amount of polyamide 6, unit g; m b is the addition amount of flame retardant, unit g; M c is the target molar mass of the flame-retardant end-modified oligomeric polyamide 6, unit g / mol; M b is the molar mass of the flame retardant, unit g / mol; k is 1.1~1.

5.

2. The method for preparing a molecular weight controllable flame-retardant end-capped modified oligomeric polyamide 6 according to claim 1, characterized in that, The initial pressure of the sealed nitrogen or inert gas environment is 0.1 MPa; after heating to 200~240℃, keep it at that temperature for 2~4 hours; after cooling to 150~180℃, keep it at that temperature for 1~3 hours.

3. The method for preparing a molecular weight controllable flame-retardant end-capped modified oligomeric polyamide 6 according to claim 1, characterized in that, The flame retardant is one or more of the following: phosphorus-containing dicarboxylic acid A, phosphorus-containing dicarboxylic acid B, (6-oxo-6H-dibenzo[c,e][1,2]oxophosphoric acid-hexane-6-yl)methyl]succinic acid, DOPO-maleic acid addition dicarboxylic acid, and DOPO-fumaric acid addition dicarboxylic acid. Among them, phosphorus-containing dicarboxylic acid A is a phosphorus-containing dicarboxylic acid obtained by the addition of itaconic acid, maleic acid, or fumaric acid with a pH-containing arylphosphinolic acid, and phosphorus-containing dicarboxylic acid B is a phosphorus-containing dicarboxylic acid obtained by the addition of itaconic acid, maleic acid, or fumaric acid with a pH-containing alkylphosphinolic acid.

4. The method for preparing a molecular weight controllable flame-retardant end-capped modified oligomeric polyamide 6 according to claim 1, characterized in that, Before mixing, polyamide 6 is vacuum dried at 90~120℃ for 16~20h.

5. The method for preparing a molecular weight controllable flame-retardant end-capped modified oligomeric polyamide 6 according to claim 1, characterized in that, The mass ratio of water to polyamide 6 is 1:1 to 1:10; the amount of flame retardant added is 5wt% to 17wt% of the amount of polyamide 6 added.

6. A flame-retardant end-capped modified oligomeric polyamide 6, characterized in that, The flame-retardant end-capped modified oligomeric amide 6 with controllable molecular weight is prepared by the method described in any one of claims 1 to 5; the flame-retardant end-capped modified oligomeric amide 6 has a number average molecular weight of 2000~8000 g / mol, a molecular weight distribution index of 1.3~1.6, a hot water extractable content of 2.0wt%~3.0wt%, and a cyclic dimer content of 0.1wt%~0.5wt%.

7. The flame-retardant end-capped modified oligomeric polyamide 6 according to claim 6, characterized in that, The flame-retardant end-capped modified oligomeric polyamide 6 has a relative viscosity of 1.1~1.7 and a melting point of 211.0~216.0℃.

8. A method for preparing a flame-retardant polyamide 6 composite material, characterized in that, Flame-retardant polyamide 6 composite material is prepared by melt blending a flame-retardant end-modified oligomeric polyamide 6 as described in claim 6 or 7 with polyamide 6 chips.

9. The method for preparing a flame-retardant polyamide 6 composite material according to claim 8, characterized in that, The mass ratio of the flame-retardant end-capped modified oligomer polyamide 6 to the polyamide 6 chip is 100:20~100; the relative viscosity of the polyamide 6 chip is 2.3~3.2, and the number average molecular weight is 2.0×10 4 ~3.5×10 4 g / mol.

10. The method for preparing a flame-retardant polyamide 6 composite material according to claim 9, characterized in that, The obtained flame-retardant polyamide 6 composite material has a tensile strength of 60~85MPa, an elongation at break of 10%~60%, a limiting oxygen index of 28%~32%, a UL-94 V-0 rating, and no ignition dripping.

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