An epoxy resin / prepolymeric benzoxazine copolymer and a method for preparing the same

Abstract

The application discloses a preparation method of an epoxy resin / prepolymerized benzoxazine copolymer. First, benzoxazine monomers are subjected to a prepolymerization reaction to form benzoxazine oligomers containing phenolic hydroxyl groups. The prepolymerization reaction can be one of a heating prepolymerization method, a catalyst prepolymerization method and a curing agent prepolymerization method. Then, the benzoxazine oligomers are subjected to a copolymerization reaction with epoxy resin to obtain the target product, the epoxy resin / prepolymerized benzoxazine copolymer. The prepolymerization reaction causes the benzoxazine monomer structure to partially undergo ring opening and generate phenolic hydroxyl groups. The phenolic hydroxyl groups can participate in the copolymerization reaction between the epoxy resin and the benzoxazine in the early curing stage, thereby improving the copolymerization degree of the epoxy resin and the benzoxazine. Compared with directly blending and polymerizing the epoxy resin and the benzoxazine, the epoxy resin / prepolymerized benzoxazine of the application has a higher curing degree, the glass transition temperature of the resin material after curing is higher, and the comprehensive performance is excellent.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of thermosetting organic polymer, in particular to an epoxy resin / prepolymerized benzoxazine copolymer and a preparation method thereof. BACKGROUND

[0002] Epoxy resin has become one of the most commonly used thermosetting resins due to its excellent comprehensive performance. It has been widely used in the fields of coatings, aerospace, composites, etc. However, the commercially available epoxy resins still need to be further improved in terms of heat resistance, thermal stability and hydrophobicity due to their own limitations.

[0003] Benzoxazine is a new type of phenolic resin containing N and O heterocyclic rings, which is prepared by the dehydration condensation reaction of phenolic compounds, amine compounds and formaldehyde under heating conditions. During the curing process, benzoxazine resin does not release small molecules and has small volume shrinkage. The cured product has high modulus and strength, high heat resistance, low dielectric, low water absorption and good flame retardant performance, and has been applied in the fields of aerospace, electronics and electrical, transportation, etc. The benzoxazine monomer forms a three-dimensional cross-linked network containing phenolic hydroxyl groups after polymerization. This phenolic hydroxyl group can copolymerize with epoxy resin. Therefore, benzoxazine monomer and epoxy resin can be copolymerized to improve the rigidity of the polymer structure, thereby improving the performance of the epoxy resin, such as improving its modulus, heat resistance, thermal stability and flame retardancy. Patent CN111269395 A discloses a benzoxazine epoxy resin copolymer containing phenolic hydroxyl groups and a preparation method thereof. The prepared copolymer resin has excellent heat resistance and thermal stability, and is suitable for use as a matrix of high-performance composites. However, the technology requires the use of specific benzoxazine containing phenolic hydroxyl groups as raw materials, which needs to be specially synthesized. The synthesis process is complex and the types are few. This method cannot be used for benzoxazine without phenolic hydroxyl groups, lacks universality, and is not suitable for large-scale application. In addition, when benzoxazine monomer is copolymerized with epoxy resin curing system, the reactivity of epoxy resin with its own curing agent is higher than that of epoxy resin with benzoxazine copolymerization. The viscosity of the copolymerization system increases rapidly, and the epoxy functional groups in the epoxy resin are greatly consumed, making it difficult for the benzoxazine monomer in the system to fully copolymerize with the epoxy resin, and even leading to a decrease in the heat resistance of the system. SUMMARY

[0004] In view of the problems of difficulty in copolymerization and narrow application range of the existing technology in the preparation process of epoxy resin and benzoxazine copolymer, the present application provides a preparation method of an epoxy resin / prepolymerized benzoxazine copolymer.

[0005] Benzoxazine as a thermosetting resin, can be ring-opening polymerization under the action of heat, catalyst or curing agent, generate phenolic hydroxyl group. In order to solve the above problems, the benzoxazine and epoxy resin copolymerization, can first be benzoxazine monomer under certain conditions of prepolymerization, by controlling the reaction conditions, make the oxazine ring ring-opening, form containing phenolic hydroxyl group of oligomer, promote the copolymerization reaction between epoxy resin and benzoxazine, so as to improve the reaction degree of epoxy resin / benzoxazine copolymer, improve the comprehensive performance of copolymer.

[0006] The preparation method of the epoxy resin / prepolymerized benzoxazine copolymer provided by the application comprises the following two steps:

[0007] S1, the benzoxazine monomer is prepolymehzed to form a benzoxazine oligomer containing phenolic hydroxyl group; the prepolymehzed method is one of heating prepolymehzed method, adding catalyst prepolymehzed method and adding curing agent prepolymehzed method.

[0008] The heating prepolymehzed method is that the benzoxazine monomer is heated to 140-180 DEG C, and the polymerization reaction is carried out for 4-7 h, and the oligomer containing phenolic hydroxyl group is obtained by controlling the polymerization reaction temperature and time.

[0009] The adding catalyst prepolymehzed method is that the catalyst is added to the benzoxazine monomer, heated to 140-160 DEG C, and the polymerization reaction is carried out for 0.5-4 h, and the oligomer containing phenolic hydroxyl group is obtained; the catalyst is selected from one of imidazole, 2-methyl-imidazole, 2-ethyl-4-methyl imidazole, oxalic acid, adipic acid, sebacic acid, p-toluene sulfonic acid and anhydride.

[0010] The adding curing agent prepolymehzed method is that the curing agent is added to the benzoxazine monomer, heated to 140-160 DEG C, and the polymerization reaction is carried out for 0.5-5 h, and the oligomer containing phenolic hydroxyl group is obtained; the curing agent is selected from one of 4, 4-diamino diphenyl methane, 4, 4'-diamino diphenyl ether, 4, 4'-diamino diphenyl sulfone, p-phenylenediamine, hydroquinone, m-phenylenediamine, m-xylene diamine, bisphenol A and bisphenol F.

[0011] S2, the benzoxazine oligomer is copolymerized with the epoxy resin to obtain the target copolymer.

[0012] The preferred polymerization method of step S2 is: adding the benzoxazine oligomer and the epoxy resin into an organic solvent to form a mixture, vacuumizing the mixture at a certain temperature to remove the solvent, and then keeping the mixture at 160℃ for 2 hours, at 180℃ for 2 hours or at 200℃ for 2 hours to obtain the epoxy resin / prepolymerized benzoxazine copolymer. A small amount of epoxy curing agent can also be added in the polymerization step to promote the polymerization reaction. The organic solvent can be one or more of acetone, butanone, chloroform, toluene, xylene, dioxane, tetrahydrofuran, N,N-dimethylformamide, etc. The vacuumizing temperature can vary according to the type of organic solvent, and is generally at a temperature lower than the boiling point of the organic solvent.

[0013] The raw material benzoxazine monomer used in the preparation method of the epoxy resin / prepolymerized benzoxazine copolymer of the present application can be any benzoxazine monomer with any molecular structure, i.e. the method is applicable to all benzoxazine monomers. For example, it can be a benzoxazine monomer with the following molecular structure:

[0014]

[0015] Taking one of the above structures as an example, the reaction principle of the three methods for pre-polymerizing the benzoxazine monomer to form a phenolic hydroxyl-containing benzoxazine oligomer is as follows:

[0016]

[0017] In step S2, the benzoxazine oligomer and the epoxy resin are subjected to a copolymerization reaction to obtain the target copolymer. Taking a certain epoxy resin and a benzoxazine oligomer as an example, the polymerization reaction principle is shown in the following reaction formula:

[0018]

[0019] Compared with the prior art, the present application has the following advantages:

[0020] (1) The present application provides a simple and convenient preparation method of an epoxy resin / prepolymerized benzoxazine copolymer. The pre-polymerization reaction causes the benzoxazine monomer structure to undergo partial ring-opening and generate a phenolic hydroxyl group, which can participate in the copolymerization reaction of the epoxy resin and the benzoxazine in the early curing stage, thereby improving the copolymerization degree of the epoxy resin and the benzoxazine. Compared with directly blending and copolymerizing the epoxy resin and the benzoxazine, the epoxy resin / prepolymerized benzoxazine has a higher curing degree, and the glass transition temperature of the resin material after curing is higher, and the comprehensive performance such as heat resistance is excellent.

[0021] (2) The overall preparation process of the present application is simple, and the equipment requirement is low, which is suitable for large-scale production.

[0022] (3) The epoxy resin / prepolymerized benzoxazine copolymer prepared by the method has excellent heat resistance, low dielectric constant and dielectric loss factor, and low water absorption rate, and has excellent performance, simple product preparation process, easy operation, and strong practicality.

[0023] Other advantages, objects, and features of the present application will be apparent from the following specification, and will be understood by persons skilled in the art. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 DMA graph of the epoxy resin / benzoxazine copolymer obtained in Example 1 and Comparative Example 1.

[0025] Figure 2 DSC graph of the epoxy resin and benzoxazine copolymer obtained in Example 2 with different prepolymerization times.

[0026] Figure 3 DSC graph of the benzoxazine before and after prepolymerization in Example 2.

[0027] Figure 4 Infrared spectrum of the benzoxazine before and after prepolymerization in Example 2.

[0028] Figure 5 DSC graph of the epoxy resin and prepolymerized benzoxazine (after prepolymerization) blend of Example 2, and the epoxy resin directly blended with benzoxazine monomers (before prepolymerization) of Comparative Example 2 before curing.

[0029] Figure 6 DSC graph of the epoxy resin and prepolymerized benzoxazine (after prepolymerization) copolymer of Example 2, and the epoxy resin directly blended with benzoxazine monomers (before prepolymerization) of Comparative Example 2 after curing. DETAILED DESCRIPTION

[0030] The preferred embodiments of the present application are described below with reference to the accompanying drawings, and it should be understood that the preferred embodiments described herein are only used to illustrate and explain the present application, and are not used to limit the present application.

[0031] Example 1

[0032] A method for preparing an epoxy resin / prepolymerized benzoxazine copolymer, the steps are as follows:

[0033] (1) The benzoxazine monomer is placed in a 140℃ air oven at room temperature and reacted for 4h or 6h to obtain a prepolymerized benzoxazine.

[0034] The structural formula of the benzoxazine monomer is as follows:

[0035] The structural formula of the benzoxazine monomer is as follows:

[0036] (2) 15 parts by weight of the prepared prepolymerized benzoxazine was added to 100 parts by weight of E44 type epoxy resin, and mixed into a uniform solution by DMF, and 25 parts by weight of curing agent DDS was added and mixed uniformly, the mixture was heated to 100 degrees Celsius, and the solvent was removed under vacuum at this temperature; then the temperature was raised for copolymerization, and an epoxy resin / DDS / prepolymerized benzoxazine copolymer was obtained. The copolymerization temperature curing process is as follows: sequentially heated to 100℃ for 1h, 140℃ for 2h, 160℃ for 2h, 180℃ for 2h.

[0037] Comparative Example 1

[0038] The same benzoxazine monomer as in Example 1 was used with E44 type epoxy resin, the difference being that the benzoxazine monomer was not prepolymerized but directly polymerized with E44 type epoxy resin. The operation is as follows:

[0039] 15 parts by weight of benzoxazine monomer was added to 100 parts by weight of E44 type epoxy resin, and mixed into a uniform solution by DMF, and 25 parts by weight of curing agent DDS was added, the mixture was heated to 100 degrees Celsius, and the solvent was removed under vacuum at this temperature; then the temperature was raised for copolymerization, and an epoxy resin / DDS / prepolymerized benzoxazine copolymer was obtained. The copolymerization temperature curing process is as follows: sequentially heated to 100℃ for 1h, 140℃ for 2h, 160℃ for 2h, 180℃ for 2h.

[0040] Figure 1 The DMA curves of the copolymers prepared in Example 1 and Comparative Example 1 can be seen from the figure, the glass transition temperature of the copolymer of benzoxazine and epoxy resin without prepolymerization is 122℃; the glass transition temperature of the copolymer of benzoxazine and epoxy resin after prepolymerization is significantly increased. The glass transition temperature of the copolymer of benzoxazine and epoxy resin after prepolymerization for 4h is greatly increased to 171℃; the glass transition temperature of the copolymer of benzoxazine and epoxy resin after prepolymerization for 6h is greatly increased to 187℃.

[0041] Example 2

[0042] A method for preparing an epoxy resin / prepolymerized benzoxazine copolymer, the steps are as follows:

[0043] (1) At room temperature, 5g of benzoxazine monomer, 0.05g of imidazole, 5g of N,N-dimethylformamide were added to a flask, a condenser was connected, and stirring and reaction were carried out at 140℃ for several hours (2h or 4h or 6h), and a prepolymerized benzoxazine was obtained.

[0044] The structural formula of the benzoxazine monomer is as follows:

[0045]

[0046] (2) The prepared prepolymerized benzoxazine solution was mixed with 10 g of E51 type epoxy resin, and the mixture was heated to 100 degrees Celsius, and the solvent was removed by vacuum at this temperature; then the temperature was raised for copolymerization; an epoxy resin / prepolymerized benzoxazine copolymer was obtained. The specific temperature raising process of the copolymerization was as follows: sequentially raising the temperature to 100 degrees Celsius for 1 hour of degassing, 140 degrees Celsius for 2 hours of heat preservation, 160 degrees Celsius for 2 hours of heat preservation, and 180 degrees Celsius for 2 hours of heat preservation.

[0047] Figure 2 The DSC curves of the epoxy resin / benzoxazine copolymers obtained by different prepolymerization times (2 h or 4 h or 6 h) of the benzoxazine monomers in this example. As can be seen from the figure, the glass transition temperature of the final benzoxazine / epoxy resin copolymer is improved after the prepolymerization of the benzoxazine monomer. Among them, the glass transition temperature of the copolymer of the benzoxazine and the epoxy resin after the prepolymerization for 4 h is the highest.

[0048] Figure 3 The DSC curves of the benzoxazine monomer before and after prepolymerization in this example. The degree of reaction of the benzoxazine monomer after prepolymerization was calculated to be 17% by the residual enthalpy. Figure 4 The infrared spectra of the benzoxazine monomer before and after prepolymerization in this example. As can be seen from the infrared spectrum after prepolymerization, the absorption peak of the oxazine ring at 942 cm -1 was weakened, indicating that the oxazine ring had undergone partial ring opening, and the absorption peak of the phenolic hydroxyl group produced after the ring opening of the oxazine ring appeared at 3400 cm -1 , proving that the prepolymerization of the benzoxazine monomer indeed occurred.

[0049] Comparative Example 2

[0050] The same benzoxazine monomer as in Example 2 was used with E51 type epoxy resin, and the difference was that the benzoxazine monomer was not prepolymerized but directly polymerized with E51 type epoxy resin. The operation was as follows:

[0051] 5 g of benzoxazine monomer was mixed with 10 g of E51 type epoxy resin, and a uniform solution was obtained by mixing with N,N-dimethylformamide, and the mixture was heated to 100 degrees Celsius, and the solvent was removed by vacuum at this temperature; then the temperature was raised for copolymerization, and an epoxy resin / benzoxazine copolymer was obtained. The specific temperature raising process of the copolymerization was as follows: sequentially raising the temperature to 100 degrees Celsius for 1 hour of degassing, 140 degrees Celsius for 2 hours of heat preservation, 160 degrees Celsius for 2 hours of heat preservation, and 180 degrees Celsius for 2 hours of heat preservation.

[0052] Figure 5DSC plots of the epoxy resin directly blended with the benzoxazine monomer (before prepolymerization) (Comparative Example 2), and the epoxy resin blended with the prepolymerized benzoxazine (after prepolymerization) (Example 2) before curing. Figure 6 DSC plots of the epoxy resin directly blended with the benzoxazine monomer (before prepolymerization) (Comparative Example 2), and the epoxy resin blended with the prepolymerized benzoxazine (after prepolymerization) (Example 2) before curing.

[0053] The above description is only the preferred embodiment of the present application, and is not intended to limit the present application in any form. Although the present application has been disclosed as above with the preferred embodiment, it is not intended to limit the present application. Any person skilled in the art can make some changes or modifications to the above disclosed technical contents to make equivalent embodiments with equivalent changes, without departing from the technical solution of the present application. Any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present application, still belong to the scope of the technical solution of the present application.

Claims

1. A method for preparing an epoxy resin / prepolymerized benzoxazine copolymer, characterized in that, It includes the following two steps: S1. The benzoxazine monomer is subjected to a prepolymerization reaction to open part of the oxazine ring, forming a benzoxazine oligomer containing phenolic hydroxyl groups; the prepolymerization reaction method is one of the following: heating prepolymerization, catalyst prepolymerization, or curing agent prepolymerization. S2. The benzoxazine oligomer and epoxy resin are added to an organic solvent to form a mixture. The mixture is then subjected to vacuum at a certain temperature to remove the organic solvent. The mixture is then kept at 160°C for 2 hours, 180°C for 2 hours, or 200°C for 2 hours to carry out a copolymerization reaction, thereby obtaining an epoxy resin / prepolymerized benzoxazine copolymer. The organic solvent is selected from one or more of acetone, butanone, chloroform, toluene, xylene, dioxane, tetrahydrofuran, and N,N-dimethylformamide.

2. The method for preparing the epoxy resin / prepolymerized benzoxazine copolymer as described in claim 1, characterized in that, The heating prepolymerization method involves heating the benzoxazine monomer to 140-180℃ and polymerizing it for 4-7 hours. By controlling the polymerization temperature and time, oligomers containing phenolic hydroxyl groups are obtained.

3. The method for preparing the epoxy resin / prepolymerized benzoxazine copolymer as described in claim 1, characterized in that, The catalyst-added prepolymerization method is as follows: a catalyst is added to the benzoxazine monomer, heated to 140-160℃, and the polymerization reaction is carried out for 0.5-4 hours to obtain an oligomer containing phenolic hydroxyl groups; the catalyst is selected from one of imidazole, 2-methyl-imidazole, 2-ethyl-4-methylimidazole, oxalic acid, adipic acid, sebacic acid, p-toluenesulfonic acid, and acid anhydrides.

4. The method for preparing the epoxy resin / prepolymerized benzoxazine copolymer as described in claim 1, characterized in that, The prepolymerization method with added curing agent is as follows: a curing agent is added to the benzoxazine monomer, heated to 140-160℃, and the polymerization reaction is carried out for 0.5-5 hours to obtain an oligomer containing phenolic hydroxyl groups; the curing agent is selected from one of 4,4-diaminodiphenylmethane, 4,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl sulfone, p-phenylenediamine, hydroquinone, m-phenylenediamine, m-phenylenediamine, bisphenol A, and bisphenol F.

5. An epoxy resin / prepolymerized benzoxazine copolymer, characterized in that, It is prepared by the preparation method described in any one of claims 1-4.

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