A phosphorus-containing imidazole oligomer flame retardant curing agent and its application in flame retardant epoxy resins

Abstract

This invention belongs to the field of flame-retardant polymer materials technology, specifically relating to a phosphorus-containing imidazole oligomer flame-retardant curing agent and its application in flame-retardant epoxy resins. The curing agent of this invention first obtains a semi-finished product through a Michael addition reaction of imidazole compounds and bio-based raw materials, and then reacts with a phosphorus-containing compound to obtain the flame-retardant curing agent. This curing agent fully utilizes the synergistic effect of multiple elements, greatly improving the flame-retardant properties of epoxy resins. Furthermore, the introduction of bio-based raw materials is of great significance for environmental protection, achieving carbon peaking and carbon neutrality, and also has good potential application value. Utilizing the special structure of oligomers, it overcomes the shortcomings of epoxy resins such as high hardness, high brittleness, and low tensile strength. Moreover, the preparation method is simple, the raw materials are widely available, and it is suitable for widespread application.

Description

Technical Field

[0001] This invention belongs to the field of flame retardant polymer materials technology, specifically relating to a phosphorus-containing imidazole oligomer flame retardant curing agent and its application in flame retardant epoxy resin. Background Technology

[0002] Since their discovery, polymers have been widely used in modern society. As an important type of thermosetting polymer, epoxy resins are widely used in construction, automotive, electronics, and aerospace fields due to their high mechanical strength, excellent adhesion, chemical resistance, and good electrical insulation.

[0003] Typically, epoxy resins can be cured under specific conditions using imidazole. Imidazole can achieve rapid curing by inducing anionic polymerization of epoxy groups in the resin. Therefore, imidazole and its derivatives are widely used as heat-latent curing agents to prepare one-component epoxy systems, which is beneficial for reducing environmental pollution and meeting the requirements of large-scale industrial production. However, unmodified imidazole has extremely high reactivity, curing epoxy resins within one or two days even at room temperature, resulting in short shelf life and poor storage stability. Furthermore, inherent flammability is another major problem that epoxy resins need to overcome; in addition to flammability, the brittleness caused by the inherently highly cross-linked structure of epoxy resins also limits their industrial applications.

[0004] Chinese patent application document (publication number: CN114539316A) discloses a phosphorus-containing imidazole compound, its preparation method and application. It uses imidazole and p-hydroxybenzyl alcohol as intermediates to react with phosphonyl chloride substances. It has disadvantages such as large addition amount and low flame retardant efficiency, and avoids its influence on the latent properties of epoxy resin.

[0005] Chinese patent application (CN114181376A) discloses a phosphorus-nitrogen flame-retardant imidazole latent curing agent, its preparation method, and its application. While exhibiting good latent properties, the large number of rigid groups obviously reduces the toughness of the epoxy resin. Therefore, developing halogen-free, environmentally friendly, highly flame-retardant, and mechanically balanced phosphorus-imidazole intrinsically flame-retardant epoxy resin materials has become an urgent need in the field of flame-retardant latent curing agents. Summary of the Invention

[0006] The purpose of this invention is to address the aforementioned problems in the prior art by proposing a phosphorus-containing imidazole oligomer flame retardant curing agent with excellent flame retardant properties, which can improve the shortcomings of epoxy resin such as high hardness, high brittleness, and low tensile strength.

[0007] The objective of this invention can be achieved through the following technical solutions:

[0008] A phosphorus-containing imidazole oligomer flame retardant curing agent, wherein the curing agent is first obtained as a semi-finished product by Michael addition reaction of imidazole compound and bio-based raw material, and then reacted with phosphorus-containing compound to obtain flame retardant curing agent.

[0009] The phosphorus-containing imidazole flame retardant curing agent prepared by this invention has an oligomer structure, which fully utilizes the synergistic effect of multiple elements to greatly improve the flame retardant performance of epoxy resin. Furthermore, with the introduction of bio-based raw materials, it is of great significance for environmental protection, achieving carbon peaking and carbon neutrality. It also has good potential application value. By utilizing the special structure of oligomers, it improves the shortcomings of epoxy resin such as high hardness, high brittleness, and low tensile strength. Moreover, the preparation method involved is simple and the raw materials are widely available, making it suitable for widespread application.

[0010] In the above-mentioned phosphorus-containing imidazole oligomer flame retardant curing agent, the imidazole compound is at least one selected from imidazole, benzimidazole, 2-benzylimidazoline, 2-cyclohexylimidazazole, 4,5-diphenylimidazazole, 2-phenylimidazazole, 4-phenylimidazazole and 5-azabenzimidazole.

[0011] As a preferred option, the chemical formula of imidazole compounds is shown below:

[0012]

[0013]

[0014] In the above-mentioned phosphorus-containing imidazole oligomer flame retardant curing agent, the bio-based raw material is at least one of maleic acid, itaconic acid, maleic anhydride, itaconic anhydride and maleimide.

[0015] As a preferred option, the chemical formula of the bio-based raw material is shown below:

[0016]

[0017] In the above-mentioned phosphorus-containing imidazole oligomer flame retardant curing agent, the phosphorus-containing compound is at least one of phosphorous acid, diethyl phosphite, trimethylolphosphine oxide, trichlorophosphine oxychloride, phenylphosphonodichlorophosphate, phenyl dichlorophosphate, phenyl dichloride, and dihydroxy-bisDOPO.

[0018] As a preferred embodiment, the chemical formula of the phosphorus-containing compound is shown below:

[0019]

[0020] In the above-mentioned phosphorus-containing imidazole oligomer flame retardant curing agent, the molar ratio of imidazole compound to bio-based raw material is 1:(0.8-1.2).

[0021] Preferably, the molar ratio of imidazole compounds to bio-based raw materials is 1:1. This invention utilizes the Michael addition reaction between the secondary amine of imidazole compounds and the double bonds in bio-based raw materials. By controlling the addition ratio of imidazole compounds to bio-based raw materials, the utilization rate of the raw materials can be greatly improved.

[0022] In the above-mentioned phosphorus-containing imidazole oligomer flame retardant curing agent, the molar ratio of the semi-finished product to the phosphorus-containing compound is 1:(0.5-2.0).

[0023] Preferably, the molar ratio of the semi-finished product to the phosphorus-containing compound is 1:2.0. The single-component curing agent prepared by this invention is a multifunctional curing agent (flame retardant, toughening and strengthening, and latent functions). Since its functionality is closely related to its molecular structure, adjusting the reaction molar ratio of the semi-finished product and the phosphorus-containing compound can yield single-component curing agents applicable to different scenarios.

[0024] In the aforementioned phosphorus-containing imidazole oligomer flame retardant curing agent, p-hydroxybenzenesulfonic acid needs to be added during the reaction of the semi-finished product with the phosphorus-containing compound.

[0025] Most preferably, the specific preparation method of the phosphorus-containing imidazole oligomer flame retardant curing agent of this application is as follows:

[0026] S1. Dissolve itaconic anhydride and 2-phenylimidazole in N,N-dimethylformamide (DMF) in a molar ratio of 1:1 to obtain a mixed solution. Place the mixed solution in a reaction vessel equipped with an oil-water separator and heat and stir for 20-30 hours at a heating temperature of 45-55℃.

[0027] S2. Add trihydroxymethylphosphine oxide and a small amount of catalyst (p-hydroxybenzenesulfonic acid) to the mixed solution obtained in step one and heat and stir for 3-8 hours at a temperature of 100-150℃.

[0028] S3. After the reaction is complete, remove the solvent and dry the phosphorus-containing imidazole oligomer flame retardant curing agent.

[0029] The present invention also provides a flame-retardant epoxy resin, which comprises the following raw materials in parts by weight: 80-120 parts epoxy resin and 8-40 parts of the phosphorus-containing imidazole oligomer flame-retardant curing agent as described in claim 1.

[0030] Preferably, the epoxy resin contains two or more epoxy groups in its molecular chain, specifically including at least one of glycidyl ether epoxy resin, glycidyl ester epoxy resin, glycidyl amine epoxy resin, linear aliphatic epoxy resin, and alicyclic epoxy resin.

[0031] The present invention also provides a method for preparing the above-mentioned flame-retardant epoxy resin, the method comprising the following steps: adding phosphorus-containing imidazole oligomer flame-retardant curing agent and epoxy resin sequentially into a reaction vessel, heating and stirring, the heating temperature being 50-90℃, the stirring time being 5-60min, until uniformly dispersed, defoaming, pouring into a mold, and heating and curing.

[0032] In the above-mentioned method for preparing a flame-retardant epoxy resin, the curing temperature is 80-180℃ and the curing time is 0.5-24h.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] 1. This invention focuses on the use of bio-based raw materials, which is of great significance for resource recycling, environmental protection, promoting carbon peaking and carbon neutrality.

[0035] 2. The flame retardant provided by this invention is an intrinsic flame retardant curing agent, which can greatly reduce problems such as flame retardant precipitation and has good durability;

[0036] 3. This invention utilizes the synergistic effect of multi-element flame retardancy and the synergistic flame retardancy effect of different phosphorus oxygen state structures to fully exert the effects of promoting char formation and inhibiting and diluting the gas phase. In epoxy resin, it has the advantages of high flame retardant efficiency and the ability to inhibit heat release and smoke release.

[0037] 4. This invention utilizes the unique properties of oligomer structure and in-situ toughening to stably maintain or even enhance the original mechanical properties without the need to add other toughening agents. Detailed Implementation

[0038] The following are specific embodiments of the present invention, which further describe the technical solution of the present invention, but the present invention is not limited to these embodiments.

[0039] In the following examples, the epoxy resin raw material used is sourced from Baling Petrochemical Company, and its grade is CYD-128. It should be noted that, unless otherwise specified, all raw materials used in the following examples are calculated by parts by weight.

[0040] Example 1:

[0041] Itaconic anhydride and 2-phenylimidazole in a molar ratio of 1:1 were dissolved in an appropriate amount of DMF and reacted at 50°C for 6 hours to obtain a semi-finished product. The semi-finished product, diethyl phosphite, and p-hydroxybenzenesulfonic acid were mixed in a molar ratio of 1.1:1 and reacted at 120°C for 6 hours. The solvent was removed, and the mixture was dried to obtain the flame retardant curing agent. The mass of p-hydroxybenzenesulfonic acid added was 0.5% of the total mass of the semi-finished product and diethyl phosphite, and this will be the standard for all subsequent examples.

[0042] Example 2:

[0043] Maleic anhydride and 2-cyclohexylimidazole in a molar ratio of 1:1 were dissolved in an appropriate amount of DMF and reacted at 50°C for 6 hours to obtain a semi-finished product. The semi-finished product, diethyl phosphite and p-hydroxybenzenesulfonic acid were mixed in a molar ratio of 1.1:1 and reacted at 120°C for 6 hours. The solvent was removed and the mixture was dried to obtain the flame retardant curing agent.

[0044] Example 3:

[0045] Maleimide and 4,5-diphenylimidazole in a molar ratio of 1:1 were dissolved in an appropriate amount of DMF and reacted at 50°C for 6 hours to obtain a semi-finished product. The semi-finished product, diethyl phosphite and p-hydroxybenzenesulfonic acid were mixed in a molar ratio of 1.1:1 and reacted at 120°C for 6 hours. The solvent was removed and the mixture was dried to obtain the flame retardant curing agent.

[0046] Example 4:

[0047] The only difference from Example 1 is that the molar ratio of the semi-finished product and trimethylolphosphine oxide is 1:0.1.

[0048] Example 5:

[0049] The only difference from Example 1 is that the molar ratio of the semi-finished product and trimethylolphosphine oxide is 1:2.0.

[0050] Application Example 1:

[0051] 15 parts of flame retardant curing agent from Example 1 and 100 parts of epoxy resin were placed in a three-necked round-bottom flask and heated and stirred at 50°C until homogeneous. The mixture was then placed under low pressure for 3 minutes to degas. While still hot, the mixture was poured into a mold preheated to 80°C and cured at 120°C and 160°C for 3 hours respectively. After curing, the mixture was allowed to cool naturally to room temperature to obtain a flame retardant epoxy resin composite material.

[0052] Application Example 2:

[0053] 20 parts of flame retardant curing agent from Example 2 and 100 parts of epoxy resin were placed in a three-necked round-bottom flask and heated and stirred at 50°C until homogeneous. The mixture was then placed under low pressure for 3 minutes to degas. While still hot, the mixture was poured into a mold preheated to 80°C and cured at 120°C and 160°C for 3 hours respectively. After curing, the mixture was allowed to cool naturally to room temperature to obtain a flame retardant epoxy resin composite material.

[0054] Application Example 3:

[0055] 25 parts of flame retardant curing agent from Example 3 and 100 parts of epoxy resin were placed in a three-necked round-bottom flask and heated and stirred at 50°C until homogeneous. The mixture was then placed under low pressure for 3 minutes to degas. While still hot, the mixture was poured into a mold preheated to 80°C and cured at 120°C and 160°C for 3 hours respectively. After curing, the mixture was allowed to cool naturally to room temperature to obtain a flame retardant epoxy resin composite material.

[0056] Application Example 4:

[0057] 20 parts of flame retardant curing agent from Example 4 and 100 parts of epoxy resin were placed in a three-necked round-bottom flask and heated and stirred at 50°C until homogeneous. The mixture was then placed under low pressure for 3 minutes to degas. While still hot, the mixture was poured into a mold preheated to 80°C and cured at 120°C and 160°C for 3 hours respectively. After curing, the mixture was allowed to cool naturally to room temperature to obtain a flame retardant epoxy resin composite material.

[0058] Application Example 5:

[0059] 20 parts of flame retardant curing agent from Example 5 and 100 parts of epoxy resin were placed in a three-necked round-bottom flask and heated and stirred at 50°C until homogeneous. The mixture was then placed under low pressure for 3 minutes to degas. While still hot, the mixture was poured into a mold preheated to 80°C and cured at 120°C and 160°C for 3 hours respectively. After curing, the mixture was allowed to cool naturally to room temperature to obtain a flame retardant epoxy resin composite material.

[0060] Table 1: Performance Test Results of Flame-Retardant Epoxy Resin Composite Materials in Application Examples 1-5

[0061]

[0062]

[0063] In summary, this invention utilizes the synergistic effect of multi-element flame retardancy and the synergistic flame retardancy of different phosphorus oxygen state structures to fully exert the effects of promoting char formation and inhibiting and diluting the gas phase. In epoxy resin, it has the advantages of high flame retardancy efficiency and the ability to inhibit heat release and smoke release. This invention utilizes the special properties of oligomer structure and in-situ toughening to stably maintain or even enhance the original mechanical properties without the need to add other toughening agents.

[0064] The embodiments described herein cover any points not exhaustively within the scope of the technical claims of this invention, as well as new technical solutions formed by equivalent substitutions of one or more technical features in the embodiments. These are all within the scope of the claims of this invention. Furthermore, in all listed or unlisted embodiments of this invention, each parameter in the same embodiment merely represents an instance (i.e., a feasible solution) of its technical solution, and there is no strict coordination or limitation relationship between the parameters. The parameters can be substituted for each other without violating axioms and the claims of this invention, unless otherwise stated.

[0065] The technical means disclosed in this invention are not limited to those described above, but also include technical solutions composed of any combination of the above technical features. The above descriptions are specific embodiments of this invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of this invention, and these improvements and modifications are also considered within the scope of protection of this invention.

[0066] The specific embodiments described herein are merely illustrative of the spirit of the invention. Those skilled in the art to which this invention pertains may make various modifications or additions to the described specific embodiments or use similar methods to substitute them, without departing from the spirit of the invention or exceeding the scope defined by the appended claims.

Claims

1. A phosphorus-containing imidazole oligomer flame retardant curing agent, characterized in that, The curing agent is first obtained as a semi-finished product by Michael addition reaction of imidazole compounds and bio-based raw materials, and then reacted with phosphorus-containing compounds to obtain a flame-retardant curing agent; The bio-based raw material is at least one of maleic acid, itaconic acid, maleic anhydride, itaconic anhydride and maleimide; The phosphorus-containing compound is at least one of phosphorous acid, diethyl phosphite, trimethylolphosphine oxide, trichloride, phenylphosphonodichlorophosphate, phenyl dichlorophosphate, and dihydroxy-bisDOPO; The chemical structural formula of dihydroxy-bisDOPO is shown below: 。 2. The phosphorus-containing imidazole oligomer flame retardant curing agent according to claim 1, characterized in that, The imidazole compound is at least one selected from imidazole, benzimidazole, 2-benzylimidazoline, 2-cyclohexylimidazazole, 4,5-diphenylimidazazole, 2-phenylimidazazole, 4-phenylimidazazole, and 5-azabenzimidazole.

3. The phosphorus-containing imidazole oligomer flame retardant curing agent according to claim 1, characterized in that, The molar ratio of imidazole compounds to bio-based raw materials is 1:(0.5-1.5).

4. The phosphorus-containing imidazole oligomer flame retardant curing agent according to claim 1, characterized in that, The molar ratio of the semi-finished product to the phosphorus-containing compound is 1:(0.5-2.0).

5. The phosphorus-containing imidazole oligomer flame retardant curing agent according to claim 1, characterized in that, p-hydroxybenzenesulfonic acid needs to be added during the reaction of the semi-finished product with phosphorus-containing compounds.

6. A flame-retardant epoxy resin, characterized in that, The flame-retardant epoxy resin comprises the following raw materials in parts by weight: 80-120 parts epoxy resin and 8-40 parts of the phosphorus-containing imidazole oligomer flame-retardant curing agent as described in claim 1.

7. A method for preparing the flame-retardant epoxy resin as described in claim 6, characterized in that, The method includes the following steps: adding phosphorus-containing imidazole oligomer flame retardant curing agent and epoxy resin sequentially into a reaction vessel, heating and stirring, the heating temperature is 50-90℃, the stirring time is 5-60min, until uniformly dispersed, defoaming, pouring into a mold and heating to cure.

8. The method for preparing a flame-retardant epoxy resin according to claim 7, characterized in that, The curing temperature is 80-180℃, and the curing time is 0.5-24h.

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