A flame-retardant recyclable plant fiber-reinforced epoxy composite material and its preparation method

Abstract

The present invention relates to a flame-retardant recyclable plant fiber-reinforced epoxy composite material and a preparation method thereof, belonging to the field of flame-retardant technology. The preparation method comprises the following steps: (1) reacting N,N,N',N'-tetraglycidyl-4,4'-diaminodiphenylmethane (TGDDM) with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) to obtain an epoxy precursor DOPO-TGDDM; (2) subjecting DOPO-TGDDM and an acid anhydride curing agent to hot pressing and curing to obtain a polyester-type epoxy-based glassy polymer (vitrimer) pre-cured sheet. (3) Alternately stacking the epoxy vitrimer pre-cured sheet and the plant fiber cloth, and putting them into a hot press to hot press to obtain a plant fiber-reinforced epoxy composite material. The prepared plant fiber-reinforced epoxy composite material has excellent intrinsic flame retardancy, smoke suppression property and degradability and recyclability.

Description

Technical Field

[0001] The invention relates to the field of flame retardant technology, and more particularly to a flame retardant recyclable plant fiber reinforced epoxy composite material and a preparation method thereof. Background Art

[0002] Plant fiber reinforced epoxy composite is a kind of plant fiber reinforced composite material. It is not only light and high-strength, but also exhibits good physical properties such as sound absorption, heat insulation, electromagnetic wave absorption and damping noise reduction. These properties make it widely used in automotive interiors, aviation doors, building pipes and other fields. However, the interface compatibility between plant fibers and epoxy resin matrix is ​​poor, resulting in reduced mechanical properties and durability of the composite material. At the same time, the composite material is highly flammable and there is a fire hazard during use. In addition, the epoxy resin matrix is ​​difficult to recycle and degrade, and the composite material will bring serious environmental pollution problems after the service ends. Therefore, the development of plant fiber reinforced epoxy composite materials with strong interface effects, flame retardant and recyclable has become a key issue to be solved.

[0003] The interfacial compatibility of plant fiber reinforced epoxy composites can be improved by plant fiber surface modification, epoxy resin matrix modification and interfacial compatibilization modification. Among them, surface modification enhances the bonding force between fiber and resin by introducing reactive groups, matrix modification optimizes mechanical properties and wettability with the help of nanofillers, and interfacial compatibilizers enhance interfacial bonding ability through chemical or physical effects. In addition, the improvement of flame retardancy of such composites has also received widespread attention. Common methods include grafting flame retardants on the surface of plant fibers, directly adding flame retardants and using intrinsic flame retardant epoxy resin matrix. Among them, although the first two can improve flame retardancy, they often face the problem of limited grafting rate or decreased mechanical properties, while the intrinsic flame retardant epoxy resin matrix can maintain the mechanical properties of the material while improving flame retardancy. In recent years, the introduction of epoxy vitrimer has provided a new strategy for the recyclability of plant fiber reinforced epoxy composites. Since the proposal of epoxy vitrimer, researchers have endowed epoxy resin chemical bonds with reversible reconstruction capabilities by introducing dynamic covalent bonds such as disulfide bonds, imine bonds, and siloxane bonds, making them processable. They have also successfully prepared biodegradable and recyclable plant fiber-reinforced epoxy composites, providing new ideas for the high performance and sustainable development of such composite materials.

[0004] Although significant progress has been made in the research of plant fiber reinforced epoxy composites, how to construct plant fiber reinforced epoxy composites with strong interface effects, flame retardancy and recyclability is a difficult point in current research, and related technologies are still relatively limited. In addition, the performance of recyclable epoxy resins after chemical recycling and remodeling is often relatively low, and how to achieve high-value utilization of degradation liquid is also a technical problem that needs to be solved urgently. Summary of the invention

[0005] In view of the deficiencies existing in the current field of epoxy resin materials, the present invention provides a flame-retardant and recyclable plant fiber-reinforced epoxy composite material and a preparation method thereof. The composite material has excellent mechanical properties, flame retardancy, and degradability and recyclability.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] A preparation method of a flame-retardant and recyclable plant fiber-reinforced epoxy composite material includes the following steps:

[0008] Step 1: React N,N,N',N'-tetraglycidyl-4,4'-diaminodiphenylmethane (TGDDM) and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) to obtain epoxy resin A;

[0009] Step 2: Mix epoxy resin A and an acid anhydride curing agent evenly, and heat and cure in a mold to obtain a pre-cured sheet;

[0010] Step 3: Alternately stack the pre-cured sheets and plant fiber cloth, and put them into a hot press for hot pressing to obtain a plant fiber-reinforced epoxy composite material.

[0011] The tertiary amine and hydroxyl structures in DOPO-TGDDM can catalyze the transesterification of epoxy resin, thereby realizing the degradability and recyclability of the material without the need to additionally add a catalyst. At the same time, the degradation liquid can be reused as a flame-retardant coating. In addition, plant fibers can achieve interfacial fusion with epoxy resin through transesterification, thereby improving the interfacial bonding force, and it is a polymer material with great potential.

[0012] Preferably, the reaction conditions in Step 1 are: under nitrogen, add N,N,N',N'-tetraglycidyl-4,4'-diaminodiphenylmethane and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide into a flask equipped with a magnetic stirrer and a condensation reflux device, and react at 150-170 °C for 6-10 h.

[0013] The beneficial effects brought by the above preference include: more complete reaction and high product purity: at a higher temperature of 150-170 °C and a sufficient reaction time of 6-10 h, N,N,N',N'-tetraglycidyl-4,4'-diaminodiphenylmethane and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide can fully react, effectively improving the yield and purity of the target product.

[0014] The reaction system is stable and controllable: reacting under nitrogen protection effectively inhibits the occurrence of oxidation side reactions, ensuring the stability and repeatability of the reaction.

[0015] Beneficial for large-scale preparation: Solvent-free reaction is adopted, which is simple, controllable, suitable for industrial production, and has good process feasibility.

[0016] Preferably, the molar ratio of N,N,N',N'-tetraglycidyl-4,4'-diaminodiphenylmethane to 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide is 1:(0.5 - 1.5).

[0017] Beneficial effects brought by the above preference include: regulating the product structure and optimizing the performance balance: flexibly regulating according to the different requirements of the reaction system for the phosphorus element content, so as to achieve the coordination and unity among heat resistance, mechanical properties and flame retardancy.

[0018] Preferably, the acid anhydride curing agent described in step 2 includes: one or a mixture of two or more of glutaric anhydride, methyltetrahydrophthalic anhydride, maleic anhydride, phthalic anhydride, nadic anhydride, itaconic anhydride, methyl nadic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, methyltetrahydrophthalic anhydride.

[0019] Beneficial effects brought by the above preference include: enhancing the versatility and process adaptability of the system: different acid anhydride types can be selected or used in combination according to application requirements, so as to achieve performance regulation and be applicable to a variety of application scenarios and process routes.

[0020] Preferably, the molar ratio of the epoxy group of epoxy resin A to the acid anhydride of the acid anhydride curing agent in step 2 is 1:(0.5 - 1).

[0021] Beneficial effects brought by the above preference include: achieving a reasonable ratio of reactive functional groups and promoting efficient curing: the epoxy group and the acid anhydride group can fully react within this molar ratio range, avoiding insufficient crosslinking degree caused by insufficient acid anhydride and preventing side reactions caused by excessive acid anhydride, thus improving the curing efficiency and product stability.

[0022] Preferably, the condition for uniform mixing in step 2 is: stirring at 100 - 150 °C for 1 - 2 min.

[0023] Preferably, the condition for heat curing in step 2 is: curing at 150 - 180 °C for 5 - 15 min.

[0024] Preferably, the condition for hot pressing in step 3 is: under 10 MPa, curing at 150 °C for 3 h, and then curing at 180 °C for 3 h.

[0025] The beneficial effects brought by the above optimizations include: improving the mixing efficiency of the system and ensuring uniform mixing of components. Stirring at 100 - 150 °C for 1 - 2 min helps reduce the viscosity of the reaction mixture, enabling rapid and thorough mixing of the epoxy resin and the anhydride curing agent, avoiding local concentration unevenness, and laying a good foundation for the subsequent curing reaction.

[0026] Precisely controlling the reaction degree to form a stable semi-cured state. Curing at 150 - 180 °C for 5 - 15 min allows for the initial cross-linking reaction between the epoxy resin and the anhydride curing agent, forming a semi-cured sheet with certain strength and thermal stability, facilitating subsequent hot pressing forming operations, and avoiding processing difficulties caused by premature complete curing.

[0027] Optimizing the cross-linking structure and material properties through staged hot pressing. First, hot pressing and curing at 150 °C and 10 MPa for 3 hours can ensure that the system cross-links gradually and releases stress at a lower temperature, forming a preliminary network structure. Subsequently, curing at 180 °C for another 3 hours helps further deep cross-linking, improves the densification and thermal stability of the material, and endows the material with excellent mechanical properties and dimensional stability.

[0028] Another object of the present invention is to provide a flame-retardant and recyclable plant fiber-reinforced epoxy composite material prepared by the preparation method of the above-mentioned flame-retardant and recyclable plant fiber-reinforced epoxy composite material.

[0029] Immersing the composite material in ethylene glycol and heating at 150 - 180 °C for 2 - 5 h can achieve the degradation of the resin. After filtration, the degradation liquid and the plant fiber are separated, and the degradation liquid can be used as a flame-retardant coating after concentration.

[0030] From the above technical solutions, compared with the prior art, the present invention has the following beneficial effects:

[0031] (1) The synthesis method of the epoxy precursor of the present invention is simple and easy to operate, without solvents and complex post-treatment processes.

[0032] (2) The synthesized epoxy precursor contains a tertiary amine structure, which can promote the epoxy curing reaction without the need for additional curing accelerators.

[0033] (3) The prepared plant fiber-reinforced epoxy composite material has excellent intrinsic flame retardancy, smoke suppression, and degradability and recyclability.

[0034] (4) The prepared plant fiber-reinforced epoxy composite material has good degradability and recyclability. Without the need for additional catalysts, the degradation liquid can be reused as a flame-retardant coating. Description of the Drawings

[0035] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only the embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on the provided drawings.

[0036] Figure 1 1H NMR spectrum of DOPO-TGDDM;

[0037] Figure 2 Flow chart for the preparation of the composite material;

[0038] Figure 3 Appearance photo of the composite material;

[0039] Figure 4 Temperature-loss factor curve of the composite material;

[0040] Figure 5 Digital photo of the composite material in the UL-94 test;

[0041] Figure 6 Heat release rate curve of the composite material;

[0042] Figure 7 Total heat release curve of the composite material;

[0043] Figure 8 Total smoke production curve of the composite material;

[0044] Figure 9 Stress relaxation curve of the epoxy resin matrix involved in the examples and comparative examples;

[0045] Figure 10 Digital photos of the composite material in Example 1 before and after chemical degradation;

[0046] Figure 11 Application of the degradation solution of the composite material in Example 1 as a flame retardant coating. Detailed implementation manners

[0047] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of them. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0048] Example 1

[0049] This embodiment provides a method for preparing a flame-retardant and recyclable plant fiber-reinforced epoxy composite, comprising the following steps:

[0050] 1. First, synthesize the epoxy precursor DOPO-TGDDM. Under nitrogen conditions, 4,4'-diaminodiphenylmethane tetraglycidylamine TGDDM (100 g, 0.2367 mol) and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide DOPO (51.16 g, 0.2367 mol) are added to a flask equipped with a magnetic stirrer and a condensing reflux device, and reacted at 155 °C for 7 h. After cooling, the final product DOPO-TGDDM is obtained.

[0051] 2. Melt and mix 12 g of the above DOPO-TGDDM and 3.25 g of glutaric anhydride at 150 °C for 1 min, pour them into a mold and cure at 150 °C for 10 min to obtain a pre-cured sheet.

[0052] 3. Alternately stack 3 pieces of the above pre-cured sheets and 2 pieces of linen fiber cloth in a mold, and cure at 150 °C for 3 h and 180 °C for 3 h under 10 MPa.

[0053] After testing, the tensile strength of the material is 92 MPa, and the glass transition temperature is 132.9 °C; it reaches V-0 level in the UL 94 vertical burning test; the limiting oxygen index (LOI) is 31.5%; the peak heat release rate (PHRR) in the cone calorimeter test is 257 kW / m 2 and the total heat release (THR) is 45.6 MJ / m 2 and the total smoke production (TSP) is 13.8 m 2 .

[0054] When the composite material is immersed in ethylene glycol, complete degradation of the resin can be achieved at 180 °C for 2 h. After filtration, the plant fiber and the degradation solution are separated. The degradation solution is concentrated and coated on wood, and reheated and cured to be used as a flame-retardant coating. When applying 200 μm, it can block the continuous burning of a butane torch at 1300 °C for more than 9 min without flame spread. While pure wood can only last for two minutes with obvious flame spread.

[0055] Example 2

[0056] This embodiment provides a method for preparing a flame-retardant and recyclable plant fiber-reinforced epoxy composite, comprising the following steps:

[0057] 1. First, synthesize the epoxy precursor DOPO-TGDDM.

[0058] 2. Melt and mix 13.43 g of DOPO-TGDDM and 3.6 g of glutaric anhydride at 150 °C with stirring for 1 min, pour into a mold and cure at 150 °C for 10 min to obtain a pre-cured sheet.

[0059] 3. Stack 4 pre-cured sheets and 3 linen fiber cloths alternately in a mold, cure at 150 °C for 3 h and 180 °C for 3 h under 10 MPa.

[0060] After testing, the tensile strength of this material is 112 MPa, the glass transition temperature is 116.6 °C; it reaches V-0 grade in UL 94 vertical burning test; the limiting oxygen index (LOI) is 30.5%; the peak heat release rate (PHRR) in the cone calorimeter test is 301 kW / m 2 , and the total heat release (THR) is 49.2 MJ / m 2 , and the total smoke production (TSP) is 12.4 m 2 .

[0061] Immerse the composite material in ethylene glycol, and all the resin can be degraded at 180 °C for 2 h. At the same time, the degradation liquid can be used as a flame retardant coating.

[0062] Comparative Example 1

[0063] 1. Melt and mix 10 g of E51 (bisphenol A type epoxy resin, epoxy value 0.51 mol / 100 g), 2.91 g of glutaric anhydride and 0.2 g of 2,4,6-tris(dimethylaminomethyl)phenol (curing accelerator) at 150 °C with stirring for 1 min, pour into a mold and cure at 150 °C for 10 min to obtain a pre-cured sheet.

[0064] 2. Stack 3 pre-cured sheets and 2 linen fiber cloths alternately in a mold, cure at 150 °C for 3 h and 180 °C for 3 h under 10 MPa.

[0065] After testing, the tensile strength of this material is 52 MPa, the glass transition temperature is 49.1 °C; there is no grade in UL 94 vertical burning test; the limiting oxygen index (LOI) is 24.6%; the peak heat release rate (PHRR) in the cone calorimeter test is 653 kW / m 2 , and the total heat release (THR) is 107.0 MJ / m 2 , and the total smoke production (TSP) is 19.9 m 2 .

[0066] Comparative Example 2

[0067] 1. Melt and mix 10 g of TGDDM and 5.4 g of glutaric anhydride at 150 °C with stirring for 1 min, then pour into a mold and cure at 150 °C for 10 min to obtain a pre-cured sheet.

[0068] 2. Alternately stack the above 3 pre-cured sheets and 2 pieces of linen fiber cloth in a mold, and cure at 150 °C for 3 h and 180 °C for 3 h under 10 MPa.

[0069] After testing, the tensile strength of this material is 59 MPa, the glass transition temperature is 89.2 °C; there is no rating in the UL 94 vertical burning test and a large amount of molten droplets are accompanied; the limiting oxygen index (LOI) is 24.2%; the peak heat release rate (PHRR) in the cone calorimeter test is 826 kW / m 2 , and the total heat release (THR) is 98.6 MJ / m 2 , and the total smoke production (TSP) is 17.4 m 2 .

[0070] Comparative Example 3

[0071] 1. Melt and mix 5 g of TGDDM, 5 g of E51 and 4.16 g of glutaric anhydride at 150 °C with stirring for 1 min, then pour into a mold and cure at 150 °C for 10 min to obtain a pre-cured sheet.

[0072] 2. Alternately stack the above 3 pre-cured sheets and 2 pieces of linen fiber cloth in a mold, and cure at 150 °C for 3 h and 180 °C for 3 h under 10 MPa.

[0073] After testing, the tensile strength of this material is 57 MPa, the glass transition temperature is 67 °C; there is no rating in the UL 94 vertical burning test; the limiting oxygen index (LOI) is 25.1%; the peak heat release rate (PHRR) in the cone calorimeter test is 793 kW / m 2 , and the total heat release (THR) is 101 MJ / m 2 , and the total smoke production (TSP) is 18.1 m 2 .

[0074] Figure 9are the stress relaxation curves of the epoxy resins involved in Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3. The relaxation time is defined as the time corresponding to when the stress (or modulus) of the material relaxes to 1 / e of the initial value. As can be seen from the figure, the epoxy resins involved in Comparative Example 1, Comparative Example 2, and Comparative Example 3 basically have no stress relaxation, while the epoxy resin prepared with TGDDM-DOPO exhibits obvious stress relaxation behavior. This is because TGDDM-DOPO contains the tertiary amine catalyst and hydroxyl structure required for the transesterification reaction, which also makes the composite material prepared with TGDDM-DOPO have excellent degradability and recyclability. At the same time, the use of TGDDM-DOPO also promotes the transesterification between plant fibers and epoxy resin, forming interfacial fusion, thereby improving the mechanical properties. In addition, the phosphorus element and hydroxyl structure in TGDDM-DOPO can promote the flame retardancy and char formation of the material, making the material have excellent smoke suppression performance ( Figure 5 ).

[0075] Since the degradation liquid of the composite material prepared with TGDDM-DOPO in Example 1 and Example 2 still contains a large amount of flame-retardant phosphorus element, the resin degradation liquid still has excellent intrinsic flame retardancy and high char formation after concentration and heat curing. Therefore, the resin degradation liquid can be used as a flame-retardant coating ( Figure 11 ), which promotes the high-value utilization of the resin degradation liquid.

[0076] In this specification, each embodiment is described in a progressive manner. The key point of each embodiment is to illustrate the differences from other embodiments. For the same or similar parts among the embodiments, reference can be made to each other. For the devices disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple. For the relevant parts, reference can be made to the description in the method section.

[0077] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be obvious to those skilled in the art. The general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to these embodiments shown herein, but will be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A preparation method of a flame-retardant recycled plant fiber reinforced epoxy composite material, characterized in that, It includes the following steps: Step 1: React N,N,N',N'-tetraglycidyl-4,4'-diaminodiphenylmethane with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide to obtain epoxy resin A; the molar ratio of N,N,N',N'-tetraglycidyl-4,4'-diaminodiphenylmethane to 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide is 1:(0.5 - 1.5); Step 2: Mix epoxy resin A and an acid anhydride curing agent evenly, and heat and cure in a mold to obtain a pre-cured sheet; the molar ratio of the epoxy groups of epoxy resin A to the acid anhydride of the acid anhydride curing agent is 1:(0.5 - 1); Step 3: Alternately stack the pre-cured sheets and plant fiber cloth, and place them in a hot press for hot pressing to obtain a plant fiber-reinforced epoxy composite material.

2. The preparation method of the flame-retardant recycled plant fiber reinforced epoxy composite material according to claim 1, characterized in that, The reaction conditions in Step 1 are: Under nitrogen, add N,N,N',N'-tetraglycidyl-4,4'-diaminodiphenylmethane and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide into a flask equipped with a magnetic stirrer and a condenser reflux device, and react at 150 - 170 °C for 6 - 10 h.

3. The preparation method of the flame-retardant recycled plant fiber reinforced epoxy composite material according to claim 1, characterized in that, The acid anhydride curing agent described in Step 2 includes: one or a mixture of two or more of glutaric anhydride, methyltetrahydrophthalic anhydride, maleic anhydride, phthalic anhydride, nadic anhydride, itaconic anhydride, methyl nadic anhydride, tetrahydrophthalic anhydride, and hexahydrophthalic anhydride.

4. The preparation method of the flame-retardant recycled plant fiber reinforced epoxy composite material according to claim 1, characterized in that, The conditions for the above-mentioned even mixing in Step 2 are: Stir at 100 - 150 °C for 1 - 2 min.

5. The preparation method of the flame-retardant recycled plant fiber-reinforced epoxy composite material according to claim 1, characterized in that, The conditions for the above-mentioned heat curing in Step 2 are: Cure at 150 - 180 °C for 5 - 15 min.

6. The preparation method of the flame-retardant recycled plant fiber reinforced epoxy composite material according to claim 1, wherein The conditions for the above-mentioned hot pressing in Step 3 are: At 10 MPa, cure at 150 °C for 3 h, and then cure at 180 °C for 3 h.

7. A flame-retardant recycled plant fiber-reinforced epoxy composite material, characterized in that, It is prepared by the preparation method of the flame-retardant recycled plant fiber-reinforced epoxy composite material according to any one of claims 1 - 6.

8. A flame-retardant recycled plant fiber-reinforced epoxy composite material according to claim 7, characterized in that, Immerse the composite material in ethylene glycol and heat it at 150 °C - 180 °C for 2 - 5 h to achieve the degradation of the resin. After filtration, separate the degradation liquid and the plant fiber. The degradation liquid can be used as a flame-retardant coating after concentration.

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