Chemically degradable curing agent with low processing viscosity and application

Abstract

The invention discloses a curing agent with low processing viscosity and capable of being chemically degraded and application, and a preparation method of the curing agent comprises the following steps: 1) carrying out a Schiff base reaction on aromatic aldehyde of p-hydroxybenzaldehyde and 2, 5-di (aminomethyl) furan to synthesize a bisphenol compound which is marked as FMA / Ar-CHO; and 2) heating the bisphenol compound FMA / Ar-CHO and excessive bismaleimide compound (MI) in a solvent for reaction to obtain FMA / Ar-CHO / MI, namely the curing agent. According to the invention, the curing agent containing both Schiff base and Diels-Alder adduct is designed, a catalytic amount of N, N '-dimethyl benzylamine is adopted as an accelerant, bisphenol A epoxy resin is cured at a lower temperature, and the bisphenol A epoxy resin shows good characteristics of repeatable hot press molding and rapid degradation in an acidic aqueous solution.

Description

Technical Field

[0001] The invention belongs to the technical field of epoxy resins, and in particular relates to a chemically degradable curing agent with low processing viscosity and an application thereof. Background Art

[0002] Epoxy resin is one of the important and widely used thermosetting plastics. Its cross-linked network and polar groups give it high mechanical properties, solvent resistance and adhesion properties, and it is widely used in the field of composite materials. However, the permanent cross-linked structure after curing makes epoxy resin and its composite materials unable to be recycled and processed after molding, resulting in resource waste and environmental damage. Although the degradation and remodeling problems of epoxy resin can be solved by dynamic covalent bonds, the types of dynamic covalent bonds limit the remodeling efficiency. Specifically, due to the exchange reaction mechanism of dynamic covalent bonds, some dynamic covalent bonds do not break during the exchange reaction, but form a metastable state similar to a ring structure, which is called a binding dynamic covalent bond; the other part of the dynamic covalent bonds exhibits the opposite behavior, which is called a dissociation dynamic covalent bond. Therefore, due to the existence of two different exchange mechanisms, the introduction of different types of dynamic covalent bonds makes epoxy resin show temperature-dependent crosslinking density changes. Compared with binding dynamic covalent bonds, the introduction of dissociation dynamic covalent bonds can make epoxy resin fully show the characteristics of thermoplasticity at high temperatures. Although the remodeling efficiency is accelerated, the use temperature is reduced. Therefore, how to make epoxy resin have both faster reshaping efficiency and be used at higher temperatures, while also being able to recover high-value fibers through chemical degradation has become a technical problem that needs to be solved urgently, especially in the application of composite materials. Summary of the invention

[0003] In view of the problems existing in the prior art, the purpose of the present invention is to provide a curing agent with low processing viscosity and chemical degradation containing two types of covalent bonds and its application. This is specifically achieved through the following technical solutions:

[0004] A chemically degradable curing agent with low processing viscosity has a structure as shown in formula (I):

[0005]

[0006] Wherein, R is an indefinite substituent on the benzene ring, selected from -OCH3 or -CH3, R' is a molecular chain connecting the bismaleimide group, and R' is selected from the group

[0007] A method for preparing a chemically degradable curing agent with low processing viscosity comprises the following steps:

[0008] 1) Aromatic aldehydes of p-hydroxybenzaldehyde and 2,5-di(aminomethyl)furan are reacted via Schiff base to synthesize bisphenol compounds, recorded as FMA / Ar-CHO:

[0009]

[0010] 2) The bisphenol compound FMA / Ar-CHO and an excess of bismaleimide compound (MI) are heated in a solvent to react to obtain FMA / Ar-CHO / MI, i.e., a curing agent:

[0011]

[0012] FMA / Ar-CHOMIFMA / Ar-CHO / MI.

[0013] Furthermore, step 1) is specifically as follows: aromatic aldehydes of the p-hydroxybenzaldehyde type and 2,5-di(aminomethyl)furan are dissolved in an ethanol solvent at a molar ratio of aldehyde group to amine group of 1:1, respectively; at room temperature, the ethanol solution of 2,5-di(aminomethyl)furan is added dropwise to the ethanol solution of the aromatic aldehyde with stirring for chemical reaction, and the rate of the dropwise addition reaction is controlled; after the dropwise addition is completed, the reaction is continued for several hours until a large amount of yellow precipitate is generated in the reaction system, the reaction is completed, a suspension is obtained, the suspension is filtered to obtain a yellow solid product, which is washed with ethanol and vacuum dried to obtain a precursor of the epoxy monomer, i.e., a bisphenol compound, recorded as FMA / Ar-CHO.

[0014] Furthermore, the dropping time is controlled within 60-120 min, and the reaction time is controlled within 8-24 h.

[0015] Furthermore, the ethanol washing times are 2-4 times, the vacuum drying temperature is 40-80°C, and the vacuum drying time is 12-24h.

[0016] Furthermore, step 2) is specifically as follows: dissolving the bisphenol compound FMA / Ar-CHO and an excess of bismaleimide compound (MI) in N,N'-dimethylformamide (DMF), and heating and stirring to react. After the reaction is completed, cooling to room temperature and slowly pouring into water to precipitate the product, filtering, washing with water and vacuum drying are performed to obtain the FMA / Ar-CHO / MI curing agent.

[0017] Furthermore, the heating and stirring temperature is 30-60°C, the reaction time is 3-5h, the vacuum drying temperature is 40-80°C, and the vacuum drying time is 12-24h.

[0018] A method for preparing an epoxy resin using the curing agent of claim 1, the method comprising the following steps:

[0019] 1) Weighing and mixing bisphenol A type liquid epoxy resin and curing agent FMA / Ar-CHO / MI according to the molar ratio of epoxy group to phenolic hydroxyl group of 1:1, adding a catalytic amount of N,N'-dimethylbenzylamine as an accelerator, stirring and mixing the three at 100-160° C. for 2-5 hours until a viscous liquid is formed;

[0020] 2) Casting the viscous liquid of step 1) into a silicone mold, then cooling it at room temperature, and demolding it to obtain a modified bisphenol A epoxy resin containing a Schiff base and a Diels-Alder adduct, achieving chemical degradation and repeated processing integration.

[0021] Application of the curing agent in the preparation of recyclable glass fiber reinforced epoxy resin composite materials.

[0022] The present invention designs a curing agent containing both Schiff base and Diels-Alder adduct, and uses a catalytic amount of N,N'-dimethylbenzylamine as a promoter to cure bisphenol A type epoxy resin at a relatively low temperature, which exhibits good repeatable hot pressing molding and rapid degradation characteristics in an acidic aqueous solution. The main mechanism is that the Schiff base gives the epoxy resin a stable cross-linked structure and can be degraded under specific conditions, while the Diels-Alder adduct gives the epoxy resin reversible cross-linking at high temperature. Through the synergistic effect of the two, the unification of low processing viscosity and rapid chemical degradation of the epoxy resin is achieved, and it can be applied to fiber composite materials that can be disassembled and recycled.

[0023] The present invention has the following beneficial effects:

[0024] 1) The curing agent constructed by the present invention through Schiff base and Diels-Alder adduct allows the epoxy resin to be repeatedly processed and chemically degraded, thereby achieving recycling;

[0025] 2) Compared with the curing agent containing only Schiff base or Diels-Alder adduct, the combination of the two can not only give the epoxy resin a lower viscosity at the processing temperature, but also give the epoxy resin chemical degradation characteristics under mild conditions;

[0026] 3) The curing agent containing Schiff base and Diels-Alder adduct of the present invention can also give epoxy resin good mechanical properties and water resistance, making up for the disadvantage that epoxy resin containing only Schiff base is afraid of water;

[0027] 4) The curing agent containing Schiff base and Diels-Alder adduct of the present invention also improves the curing process, so that during the ring-opening addition of the epoxy resin with the curing agent, the thermal reversibility of Diels-Alder gives it fluidity at the curing temperature, thereby improving the efficiency of the epoxy ring-opening reaction and reducing the operation time of the curing stage. DETAILED DESCRIPTION

[0028] The present invention is further described below in conjunction with specific embodiments.

[0029] Example 1: Preparation of Schiff base bisphenol

[0030] 1) Aromatic aldehydes of the p-hydroxybenzaldehyde class and 2,5-di(aminomethyl)furan are dissolved in an ethanol solvent at a molar ratio of aldehyde group to amine group of 1:1. At room temperature, the ethanol solution of 2,5-di(aminomethyl)furan is added dropwise to the ethanol solution of the aromatic aldehyde with stirring to carry out a chemical reaction. The dropping time is controlled within 90 minutes. After the dropwise addition is completed, the reaction is continued for 12 hours until a large amount of yellow precipitate is generated in the reaction system, and the reaction is completed.

[0031] 2) The suspension obtained in step 1) was filtered to obtain a yellow solid product, which was washed with ethanol three times and vacuum dried at 60° C. for 12 h to obtain a precursor of an epoxy monomer, denoted as FMA / Ar-CHO, with the reaction formula being:

[0032]

[0033] Wherein, R is an indefinite substituent on the benzene ring, selected from -OCH3 or -CH3.

[0034] Example 2: Preparation of Schiff base curing agent containing Diels-Alder adduct The obtained precursor FMA / Ar-CHO and excess bismaleimide compound (MI) were dissolved in N,N'-dimethylformamide (DMF) and heated and stirred at 40°C. After reacting for 4 hours, the mixture was cooled to room temperature and slowly poured into water to precipitate the product, which was filtered, washed with water and vacuum dried at 60°C for 12 hours to obtain FMA / Ar-CHO / MI curing agent, and the reaction formula was:

[0035]

[0036] FMA / Ar-CHOMIFMA / Ar-CHO / MI

[0037] Wherein, R is an indefinite substituent on the benzene ring, selected from -OCH3 or -CH3, R' is a molecular chain connecting the bismaleimide group, and R' is selected from the group

[0038] Example 3: Preparation of the prepared FMA / Ar-CHO / MI curing agent and bisphenol A epoxy resin cured sample

[0039] 1) Weigh and mix bisphenol A type liquid epoxy resin and curing agent FMA / Ar-CHO / MI according to the molar ratio of epoxy group to phenolic hydroxyl group of 1:1, and add catalytic amount of N,N'-dimethylbenzylamine as an accelerator. Stir and mix the three under heating conditions until a viscous liquid is formed.

[0040] 2) Casting the stirred viscous liquid into a silicone mold, cooling it at room temperature, and demolding it to obtain a modified bisphenol A epoxy resin containing a Schiff base and a Diels-Alder adduct, thereby achieving chemical degradation and repeated processing integration.

[0041] Wherein, in step 1), the amount of N,N'-dimethylbenzylamine added is controlled at 0.5-2 mol%, and the optimal amount is 1 mol%; the heating temperature is controlled at 100-160°C, the heating time is controlled at 2-5h, the optimal heating temperature is 140°C, and the optimal heating time is 3h.

[0042] Example 4: Preparation of recyclable glass fiber reinforced epoxy resin composites using FMA / Ar-CHO / MI curing agent

[0043] 1) Pretreatment of glass fiber cloth: Cut several pieces of untwisted coarse sand glass fiber non-woven fabric of specific size and place them in a 100-150℃ oven to dry for 6-12h.

[0044] 2) After mixing bisphenol A epoxy resin and FMA / Ar-CHO / MI curing agent, prepare epoxy resin prepolymer according to the above preferred curing steps. Several pieces of glass fiber non-woven fabrics are impregnated with epoxy resin prepolymer for 30-60 μm to obtain glass fiber non-woven fabric prepreg.

[0045] 3) After stacking several glass fiber non-woven fabric prepregs in parallel according to the fiber texture, hot pressing is performed at 5 MPa and 100-160° C. for 10-20 minutes to obtain a complete glass fiber reinforced epoxy resin composite material.

[0046] The process of degrading and regenerating the glass fiber reinforced epoxy resin composite material through an acidic solution is specifically as follows:

[0047] 1) Degradation of glass fiber reinforced epoxy resin composite material: The composite material is placed in an acidic solution of HCl / THF with a volume ratio of 1:1 and heated until most of the resin is degraded and dissolved in the degradation solution to obtain a bare glass fiber non-woven fabric, wherein the HCl concentration in the acidic solution is controlled at 0.1-1 mol / L and the heating temperature is controlled at 30-60° C.; the optimal HCl concentration is 0.5 mol / L and the optimal heating temperature is 45° C.

[0048] 2) Regeneration of degradation liquid and resynthesis of glass fiber reinforced epoxy resin: After the degradation liquid is added to the NaOH solution for precipitation, it is filtered to obtain a viscous yellow solid. After drying the solid at 100°C, it is ground into fine powder and spread on the recovered glass fiber non-woven fabric. Similarly, multiple layers of glass fiber non-woven fabrics are stacked, and epoxy resin fine powder of the same quality is spread between each layer. Finally, hot pressing is performed according to the preferred method of the above step 3). Among them, the concentration of NaOH solution is controlled at 1-3mol / L, the drying time of the yellow solid is controlled at 2-5h, and the powder particle size is controlled at 60-200 mesh. The optimal NaOH concentration is 2mol / L, the optimal drying time is 3h, and the optimal powder particle size is 100 mesh.

[0049] 3) The performance of the obtained fiber-reinforced epoxy resin and the impact of recycling on it: Finally, a high-strength and high-modulus recyclable glass fiber-reinforced epoxy resin with a tensile modulus of about 5 GPa can be obtained; and after the same composite material is repeatedly processed three times, the tensile strength only decreases by 10%, showing good recyclability and repeated processing performance.

Claims

1. A curing agent with low processing viscosity and chemical degradability, characterized in that: It has a structure as shown in formula (I): Wherein, R is an indefinite substituent on the benzene ring, selected from -OCH3 or -CH3, R' is a molecular chain connecting the bismaleimide group, and R' is selected from the group 2. The method for preparing a chemically degradable curing agent with low processing viscosity as claimed in claim 1, characterized in that: The following steps are involved: 1) Aromatic aldehydes of p-hydroxybenzaldehyde and 2,5-di(aminomethyl)furan are reacted via Schiff base to synthesize bisphenol compounds, recorded as FMA / Ar-CHO: 2) The bisphenol compound FMA / Ar-CHO and an excess of bismaleimide compound (MI) are heated in a solvent to react to obtain FMA / Ar-CHO / MI, i.e., a curing agent: FMA / Ar-CHOMIFMA / Ar-CHO / MI.

3. The method for preparing a chemically degradable curing agent with low processing viscosity as claimed in claim 2, characterized in that: Step 1) is specifically as follows: aromatic aldehyde of the p-hydroxybenzaldehyde type and 2,5-di(aminomethyl)furan are dissolved in an ethanol solvent at a molar ratio of aldehyde group to amine group of 1:1, respectively; at room temperature, the ethanol solution of 2,5-di(aminomethyl)furan is added dropwise to the ethanol solution of the aromatic aldehyde with stirring for chemical reaction, and the rate of the dropwise addition reaction is controlled; after the dropwise addition is completed, the reaction is continued for several hours until a large amount of yellow precipitate is generated in the reaction system, the reaction is completed, a suspension is obtained, the suspension is filtered, a yellow solid product is obtained, and after washing with ethanol and vacuum drying, a precursor of the epoxy monomer, i.e., a bisphenol compound, is obtained, which is recorded as FMA / Ar-CHO.

4. The method for preparing a curing agent with low processing viscosity and chemical degradation as claimed in claim 3, characterized in that: The dropping time is controlled within 60-120 min, and the reaction time is controlled within 8-24 h.

5. The method for preparing a curing agent with low processing viscosity and chemical degradation as claimed in claim 3, characterized in that: The number of ethanol washings is 2-4 times, the vacuum drying temperature is 40-80°C, and the vacuum drying time is 12-24h.

6. The method for preparing a chemically degradable curing agent with low processing viscosity as claimed in claim 2, characterized in that: Step 2) is specifically as follows: dissolving the bisphenol compound FMA / Ar-CHO and an excess of bismaleimide compound (MI) in N,N'-dimethylformamide (DMF), and heating and stirring to react. After the reaction is completed, cooling to room temperature and slowly pouring into water to precipitate the product, filtering, washing with water and vacuum drying to obtain the FMA / Ar-CHO / MI curing agent.

7. The method for preparing a chemically degradable curing agent with low processing viscosity as claimed in claim 6, characterized in that: The heating and stirring temperature is 30-60°C, the reaction time is 3-5h, the vacuum drying temperature is 40-80°C, and the vacuum drying time is 12-24h.

8. A method for preparing epoxy resin using the curing agent of claim 1, characterized in that: The method comprises the following steps: 1) Weighing and mixing bisphenol A type liquid epoxy resin and curing agent FMA / Ar-CHO / MI according to the molar ratio of epoxy group to phenolic hydroxyl group of 1:1, adding a catalytic amount of N,N'-dimethylbenzylamine as an accelerator, stirring and mixing the three at 100-160° C. for 2-5 hours until a viscous liquid is formed; 2) Casting the viscous liquid of step 1) into a silicone mold, then cooling it at room temperature, and demolding it to obtain a modified bisphenol A epoxy resin containing a Schiff base and a Diels-Alder adduct, achieving chemical degradation and repeated processing integration.

9. Use of the curing agent according to claim 1 in preparing recyclable glass fiber reinforced epoxy resin composite materials.

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