Preparation method and application of double-dynamic-bond modified epoxy resin and modified wood

Abstract

The invention discloses a preparation method and application of double-dynamic-bond modified epoxy resin and modified wood, and belongs to the technical field of wood modification. The preparation method specifically comprises the following steps: (1) synthesizing a flame retardant VAP containing a Schiff base bond; (2) synthesis of a flame retardant BDB containing boron ester bonds; (3) preparing double dynamic bond modified epoxy resin; and preparing the modified epoxy resin impregnated wood. The double-dynamic-bond modified epoxy resin has dynamic characteristics, solves the problem of brittleness of common waterborne epoxy resin, and enhances the toughness of the epoxy resin; moreover, reversible fracture and rearrangement of the dynamic bonds endow the material with plasticity, so that the material can be reprocessed and formed, and meanwhile, due to the characteristic that the dynamic bonds are fractured under specific stimulation, the problem that thermosetting resin cannot be recycled and reprocessed is solved, the service life of wood is prolonged, and the application field of wood-based materials is expanded.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of wood modification, and more particularly to a double dynamic bond modified epoxy resin and a preparation method and application of modified wood. BACKGROUND

[0002] Wood, as a kind of light weight, high strength and environmentally friendly material, has attracted more and more attention due to its additional advantage of carbon sequestration. However, due to the scarcity of high-quality hardwood resources, the prohibition of natural forest logging in China, and the restriction of wood export in foreign countries, the contradiction between supply and demand of wood is becoming increasingly prominent. Fast-growing wood from plantations with short growth cycle plays an increasingly important role in wood supply. However, fast-growing wood usually has low dimensional stability and poor mechanical properties. Therefore, it is of great significance to modify fast-growing wood from plantations to improve its physical and mechanical properties and expand its utilization value.

[0003] Wood modification is to improve or change the physical, mechanical and chemical properties and structural characteristics of wood by physical or chemical methods, to improve the mechanical strength and dimensional stability of wood, or to make it have corrosion resistance, flame resistance and wear resistance, which has been widely used to improve the performance of wood. However, it is still a great challenge to simultaneously improve the strength and toughness of wood.

[0004] The commonly used physical modification methods at present mainly include wood heat treatment and wood compression treatment. Heat treatment can improve the dimensional stability of wood and eliminate or slow down the release of internal stress of wood, but after heat treatment, the wood will have surface and internal cracks, and the color will change greatly. Compression treatment can improve the mechanical properties of wood, but it will absorb moisture and rebound in a humid environment, resulting in unstable size.

[0005] The commonly used chemical modification methods at present mainly include immersing chemicals into the interior of wood without destroying the wood, so as to make the wood overcome the problems of poor dimensional stability and low density. The main chemical modification methods for improving fast-growing wood from plantations include acetylation, furfurylation, in-situ polymerization, paraffin modification, silanization and mineralization. For example, the polymerization of vinyl monomers such as styrene (St) and methyl methacrylate (MMA) can greatly improve the dimensional stability of wood, but the use of organic solvents is contrary to the current pursuit of green modification technology, and the expensive manufacturing cost also makes it difficult to be widely used in a long time in the future.

[0006] In addition, thermosetting resins are also commonly used for wood modification, including urea-formaldehyde resin, phenol-formaldehyde resin and melamine-formaldehyde resin. However, urea-formaldehyde resin and melamine-formaldehyde resin have problems such as short curing time, difficult to control the curing process, easy to age during use, etc.; phenol-formaldehyde resin has the disadvantages of high reaction temperature, dark color of the treated material, etc., and these resins will release formaldehyde during production and use, which is harmful to the environment. Moreover, phenol-formaldehyde resin and urea-formaldehyde resin will cause small molecules to invade the cell wall and then cross-link to form a three-dimensional structure, hindering the movement of macromolecules and leading to a decrease in the toughness of wood.

[0007] Waterborne epoxy resin is widely used in material bonding and coating corrosion prevention due to its excellent chemical stability, heat resistance and high mechanical strength. Due to the characteristic of no free formaldehyde release during use, waterborne epoxy resin is more commonly used in wood functional coatings. However, so far, there are few studies on the application of waterborne epoxy resin in the modification of fast-growing wood impregnation.

[0008] At the same time, although extensive research has been conducted on wood modification, the recycling and utilization of modified wood have rarely been addressed. Some modifiers form a three-dimensional network structure, which makes the recycling of modified wood very difficult due to its non-meltable or insoluble structure. It is more environmentally friendly to recycle or recycle modified wood than to degrade or incinerate it. For example, although biodegradable plastics can return carbon and nutrients to the soil, the energy and resources related to the production of plastics have actually been lost.

[0009] Currently, useful substances such as lignin can be extracted from waste wood or modified wood, or it can be crushed and added to adhesives to make artificial boards, or used as auxiliary materials for other composite materials, or dissolved in acid solution to make wood pulp, or incinerated. However, these methods usually require additional chemicals. If modified wood can be recycled without adding additional chemicals, not only can its application field be broadened, but also its life cycle can be greatly extended, making it the most environmentally friendly wood.

[0010] Therefore, how to simultaneously modify and recycle wood is a problem that those skilled in the art need to solve. SUMMARY

[0011] Therefore, the purpose of the present application is to provide a double dynamic bond modified epoxy resin and a preparation method and application of modified wood to solve the problems in the prior art.

[0012] In order to achieve the above purpose, the technical scheme adopted by the present application is as follows:

[0013] A preparation method of a double dynamic bond modified epoxy resin, specifically comprising the following steps:

[0014] Synthesis of flame retardant VAP containing Schiff base bond

[0015] Methanol and hydrochloric acid were heated, gamma-aminopropyl triethoxysilane was added, and the reaction was carried out at a high temperature, and then the reaction was cooled, poured into tetrahydrofuran to form a white precipitate, and the white precipitate was washed and vacuum dried to obtain a white powder OA-POSS; the OA-POSS was mixed with vanillin in ethanol, and the reaction was carried out at a high temperature again, and then the reaction was dried to obtain a yellow VAP solid powder;

[0016] Synthesis of flame retardant BDB containing boron ester bond

[0017] 1,4-benzenediboronic acid and 1-thioglycerol were dissolved in a mixed solvent of tetrahydrofuran and water, magnesium sulfate was added, and the mixture was stirred, filtered and concentrated, and then washed to obtain a white BDB solid product;

[0018] Preparation of a double dynamic bond modified epoxy resin

[0019] The epoxy resin, the yellow VAP solid powder and the white BDB solid product were mixed uniformly to obtain the double dynamic bond modified epoxy resin.

[0020] Further, in the above step (1), the amount ratio of methanol, hydrochloric acid, gamma-aminopropyl triethoxysilane and tetrahydrofuran was 100 mL: 15 mL: 15.2 mL: 200 mL; the amount ratio of OA-POSS, vanillin and ethanol was 2.64 g: 3.65 g: 50 mL.

[0021] Further, in the above step (1), the temperature was heated to 60℃; the temperature of the reaction at a high temperature was 90℃, and the time was 18h; the temperature was cooled to room temperature; the reagent for washing was anhydrous ethanol; the temperature of vacuum drying was 40℃; the temperature of the reaction at a high temperature again was 60℃, and the time was 3h; the temperature of drying was 40℃.

[0022] Further, in the above step (2), the amount ratio of 1,4-benzenediboronic acid, 1-thioglycerol, tetrahydrofuran, water and magnesium sulfate was 18.1 mmol: 37.1 mmol: 80 mL: 0.1 mL: 5.0 g.

[0023] Further, in the above step (2), the temperature of stirring was room temperature, and the time was 24h; the reagent for washing was dichloromethane.

[0024] Further, in the above step (3), the amount ratio of the epoxy resin, the yellow VAP solid powder and the white BDB solid product was 100 g: 5 g: 7 g.

[0025] The application also claims a double dynamic bond modified epoxy resin prepared by the above preparation method.

[0026] The application also claims the use of the double-dynamic bond modified epoxy resin prepared by the above preparation method in wood treatment.

[0027] A preparation method of a flame-retardant epoxy modified wood, specifically comprising the following steps:

[0028] (1) Pretreatment of the modified wood

[0029] The fast-growing poplar is soaked in water, microwave-heated after being taken out, and freeze-dried to obtain dried poplar wood.

[0030] (2) Preparation of the flame-retardant epoxy modified wood

[0031] The dried poplar wood is added into the double-dynamic bond modified epoxy resin prepared by the above preparation method for vacuum impregnation, pressure maintaining, vacuum sealing and heating to obtain the flame-retardant epoxy modified wood.

[0032] Further, in the step (1), the soaking time is 1 h; the microwave power for microwave heating is 500 W, and the time is 18 min; the density of the dried poplar wood is 0.48-0.52 g / cm 3 .

[0033] The above further beneficial effect is that the microwave oven drying method will close some pores originally existing in the water treatment process, and therefore the freeze-drying method is selected to solidify the pores of the microwave-treated sample, and the sample is frozen and then put into a freeze dryer for treatment until it is absolutely dry, so as to minimize the influence of wood variability.

[0034] Further, in the step (2), the vacuum impregnation time is 10 min; the pressure for pressure maintaining is 6 kPa, and the time is 60 min; the temperature for vacuum sealing and heating is 30℃, and the time is 2 h.

[0035] The above further beneficial effect is that the vacuum pressure impregnation method is used to immerse the modifier into the fast-growing wood, and the size stability, mechanical properties and surface properties of the fast-growing wood before and after the modification treatment are studied to obtain fast-growing wood with good performance, so as to broaden the application of the fast-growing wood in the fields of building and decoration. The vacuum sealing and heating prevent the volatilization of small molecules in the double-dynamic bond modified epoxy resin and avoid the inhibition of oxygen on the free radical polymerization.

[0036] According to the above technical solution, compared with the prior art, the application has the following beneficial effects:

[0037] The present application adopts a waterborne epoxy resin which can be cured at room temperature as a modifier, the modified waterborne epoxy resin is a nanocomposite containing dynamic bonds, which is called epoxy solid plastic sol and has dynamic characteristics. This characteristic solves the brittleness problem of ordinary waterborne epoxy resin and enhances its toughness. Moreover, the development of dynamic bonds effectively solves the recycling problem of modified wood, because the reversible rupture and rearrangement of dynamic bonds endow the material with plasticity, which can be reprocessed into shape, and due to the characteristic of dynamic bonds breaking under certain stimulation, the material can also be degraded and recycled. BRIEF DESCRIPTION OF DRAWINGS

[0038] Figure 1 Synthesis route and impregnation process diagram of double dynamic bond modified epoxy resin;

[0039] Figure 2 Scanning electron microscope images of Ctrl (a), freeze-dried wood (b) and microwave freeze-dried wood (c);

[0040] Figure 3 Fourier transform infrared spectra (a) and XPS (b) of dynamic Schiff base and boron ester bond;

[0041] Figure 4 Physical properties of modified epoxy resin and modified epoxy resin impregnated wood: (a) weight gain rate (WPG), volume filling rate (VFP), porosity, (b1-b2) scanning electron microscope of V10-P120-DB-LA, (c) BET, (d) DSC, (e-g) DMA, (i) nanoindentation;

[0042] Figure 5 Distribution of micropores in impregnated wood in different two-dimensional planes;

[0043] Figure 6 Three-dimensional Micro-CT of bulk phase, resin phase and pore phase and pore distribution analysis based on pore phase;

[0044] Figure 7 Bending strength test schematic diagram (a), stress-strain curve (b), wood fracture modulus (MOR) and elastic modulus (MOE) (c-d), fracture surface schematic diagram (e), wood impact strength (f), comparison of mechanical properties in this study and literature (g);

[0045] Figure 8 UL-94 test results and LOI (j) of a. Ctrl; b. 3% VAP; c. 5% VAP; d. 7% VAP; e. 3% BDB; f. 5% BDB; g. 7% BDB; h. 9% BDB; i. 5% VAP+7% BDB;

[0046] Figure 9 (a1) Cone calorimeter, (a2) Heat release rate of EP-coated wood, (a3) ​​Total heat released by EP-coated wood, (a4) Smoke production rate of EP-coated wood;

[0047] Figure 10 (a) Schematic diagram of the overall heat transfer process of the sample, temperature curve along the x-axis (top view); (be) Infrared thermal imaging test; (b) EP, (c) 5% VAP-EP, (d) 7% BDB-EP, (e) Infrared thermal images of VAP+BDB-EP;

[0048] Figure 11 Raman spectral analysis of (a) wood residue impregnated with pure epoxy resin, (b) wood residue impregnated with boron ester bond modified epoxy resin, (c) wood residue impregnated with imine bond modified epoxy resin and (d) wood residue impregnated with boron ester and imine bond co-modified epoxy resin.

[0049] Figure 12 This is a diagram illustrating the flame retardant mechanism.

[0050] Figure 13 (a1) Square material impregnated with water-based epoxy resin, (a2) Compression of square material impregnated with water-based epoxy resin, (a3) ​​MOR and MOE of square material before and after compression, (b1) Poplar veneer of 2cm×2cm×30cm, (b2) Poplar veneer impregnated with epoxy resin, (b3) MOR and MOE of poplar veneer impregnated with epoxy resin after hot pressing in a hot press.

[0051] Figure 14 Hot pressing for dynamic borosilicates and Schaeff base bonds in different proportions;

[0052] Figure 15 A schematic diagram illustrating the recyclability of impregnated wood;

[0053] Figure 16 For the waterproofing effect of impregnated wood: (a) water absorption rate, (b) thickness expansion rate, (c) veneer before and after water impregnation, (d) anti-expansion effect, (e) equilibrium moisture content, (f) cube before and after water impregnation. Detailed Implementation

[0054] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0055] Example 1

[0056] A method for preparing a dual-dynamic bond modified epoxy resin, as shown in Figure 1 , specifically comprising the following steps:

[0057] (1) Synthesis of flame retardant VAP containing Schiff base bond

[0058] In a 250 mL three-necked round bottom flask equipped with a magnetic stirrer, a burette and a reflux condenser, 100 mL of methanol and 15 mL of hydrochloric acid were added, heated to 60°C, then 15.2 mL of γ-aminopropyl triethoxysilane (KH-550) was slowly added, the mixture was kept at 90°C for 18 h, after cooling to room temperature, the product was poured into 200 mL of tetrahydrofuran to form a white precipitate, then washed with anhydrous ethanol, and dried under vacuum at 40°C to obtain white powder OA-POSS, 2.64 g of OA-POSS was mixed with 3.65 g of vanillin in 50 mL of ethanol, reacted at 60°C for 3 h, then dried at 40°C to remove volatile matter, to obtain yellow VAP solid powder;

[0059] (2) Synthesis of flame retardant BDB containing boron ester bond

[0060] First, 18.1 mmol of 1,4-benzene diboronic acid and 37.1 mmol of 1-thioglycerol were dissolved in 80 mL of a mixed solvent of tetrahydrofuran and 0.1 mL of water, then 5.0 g of magnesium sulfate was added, the reaction system was stirred at room temperature for 24 h, then filtered and concentrated, the obtained solid was repeatedly washed with dichloromethane to obtain white BDB solid product;

[0061] (3) Preparation of dual-dynamic bond modified epoxy resin

[0062] 100 g of epoxy resin, 5 g of yellow VAP solid powder and 7 g of white BDB solid product were mixed uniformly to obtain a dual-dynamic bond modified epoxy resin.

[0063] Example 2

[0064] A method for preparing flame-retardant epoxy modified wood, as shown in Figure 1 , specifically comprising the following steps:

[0065] (1) Pretreatment of wood

[0066] The fast-growing poplar was soaked in distilled water for 1 h, then the excess water on the surface was wiped off with a wet filter paper, and then heated in a 500 W microwave oven for 18 min, and freeze-dried to obtain dry poplar with a density of 0.48-0.52 g / cm 3 ;

[0067] (5) Preparation of flame-retardant epoxy modified wood

[0068] Dry poplar wood was added to the double dynamic bond modified epoxy resin of Example 1 and vacuum impregnated for 10 min, followed by holding at a pressure of 6 kPa for 60 min. After impregnation, the residual resin on the sample surface was wiped off with lint-free paper. The impregnated sample was placed in a vacuum-sealed bag and heated at 30°C for 2 h to obtain flame-retardant epoxy modified wood.

[0069] Performance testing

[0070] 1. Experimental Procedure

[0071] The dynamic properties and recyclability of the double dynamic bond modified epoxy resin prepared in Example 1 were tested.

[0072] The mechanical properties, flame retardant properties, and recyclability of the flame-retardant epoxy-modified wood prepared in Example 2 were tested.

[0073] 2 Results and Discussion

[0074] 2.1 Wood Pretreatment: Microwave Freeze-drying

[0075] Figure 2 The results showed that after microwave treatment, the cell wall structure of the wood exhibited significant shrinkage and deformation, and cracks were produced. This indicates that the intense steam movement caused by microwaves disrupts the weak structure of the cell walls, increases the contact between the wood and the impregnation solution, and thus improves the impregnation effect.

[0076] During the pre-microwave treatment, the wood was exposed to moisture. Moisture is a significant factor affecting the pore structure of wood cell walls. The pores in the wood cell walls provide space for moisture, and moisture, as a polar swelling solvent, also reacts on the wood cell walls, altering their pore structure. The wood cell walls significantly thickened after moisture treatment, indicating that as moisture entered the wood cell walls, some hydrogen bonds within the cell walls were opened, microfilaments swelled with moisture, and the cell wall space increased. However, the effect of moisture on wood porosity is reversible; the collapsed pores inside the re-moistened wood will recover, causing the pore size and specific surface area to increase. Oven drying methods can close some pores that existed before moisture treatment. Therefore, this study chose freeze-drying to solidify the pores of the wet-treated samples. The samples were first frozen and then treated in a freeze dryer until they were completely dry.

[0077] 2.2 Characterization of dynamic Schiff bases and boronic ester bonds

[0078] The infrared spectrum of the dynamic bond is shown in Figure 3 (a) Dynamic boron ester bond modified epoxy resin is formed by the chemical reaction of the thiol of BDB and the epoxy group of o-CFER to form a boron-containing o-CFER network. Fourier transform infrared spectroscopy was used to determine -SH and BO at 2568 cm⁻¹. -1 and 1360cm -1stretching vibration of the C=0 group in the imine group usually produces an infrared absorption peak between 1650-1670 cm -1 , while the C=N stretching vibration produces an absorption peak between 1400-1450 cm -1 . In addition, the amino (-NH2) functional group in the imine group shows a distinct absorption peak in the infrared spectrum, and the N-H stretching vibration is usually between 3000-3500 cm -1 .

[0079] Figure 3 The XPS spectrum of the dynamic boronate ester bond in (b1)-(b2) showed that the C1s peak could be decomposed into four components at 288.5, 286.3, and 286.5 eV, corresponding to -C-C-, -C-SH-, and -C-O-C bonds, respectively. The C=0 group was detected at 532.5 eV in the O1s spectrum.

[0080] Figure 3 The XPS spectrum of the Schiff base in (c1)-(c3) showed that the C1s peak was decomposed into four components at 284.4, 286.0, and 288.7 eV, corresponding to -C-C-, -C-O-C, and -C-Si- bonds, respectively. In addition, the N1s spectrum showed characteristics at 398.3 and 401 eV, which were attributed to -C-N- and -C=N- groups. In addition, Si-O-Si, P-O-C, and C-O-C groups were detected at 533.7, 532.2, and 532.3 eV in the O1s spectrum. These results confirmed the synthesis of dynamic Schiff base and boronate ester bonds.

[0081] 2.3 Analysis of the physical and microstructure of dynamic bond modified epoxy nanomaterials and impregnated wood

[0082] Small wood blocks with dimensions of 2 cm x 2 cm x 2 cm were treated using different impregnation methods. Dynamic epoxy resin impregnation can effectively reduce the porosity of poplar from the initial 70% to 25%, while the WPG of V10-A120 is only 60%. The addition of wetting agent (LA) diethylene glycol butyl ether (DB) can reduce the surface tension of the liquid, enhance the wetting and penetration of the impregnating liquid in the wood pores, and then penetrate into the interior of the wood. After V10-A120-DB-LA treatment, the WPG can reach 90%. Without external force, the resin enters the wood by capillary action and diffusion, which is limited. Therefore, the impregnation pressure plays a crucial role in the impregnation effect. For V10-P120-DB-LA, the WPG increases significantly to 110%. This is because the resin enters under the action of pressure difference, and the gas is extracted from the wood during the evacuation process, providing more space for resin filling. With the increase of impregnation time, impregnation pressure and impregnation additives, the WPG shows a linear growth. This shows that the synergistic effect of vacuum, pressure and additives effectively improves the WPG of wood.

[0083] In addition to WPG, the value of VFP is also noteworthy. As shown in (a) of Figure 4 After treatment with V10-A120, the resin only filled 65% of the pores. After adding diethylene glycol butyl ether (DB) and wetting agent (LA), the resin filled 70% of the pores, and after increasing the pressure, the resin filled 73% of the pores treated with V10-P120-DB-LA. Scanning electron microscopy (SEM) was used to observe the microstructure and morphology of the modified wood. As shown in (b1)-(b2) of Figure 4 The intercellular space of the impregnated wood appears partially filled and completely filled, which clearly shows that the dynamic epoxy resin has completely entered the vessel wall, and there are some obvious lumps around the vessel. In addition, the cell cavity is also fully filled with dynamic epoxy resin and is closely combined with the inner wall. The above results show that the dynamic epoxy resin can be effectively fixed inside the poplar.

[0084] As can be seen from (c) of Figure 4 The adsorption-desorption data obtained by BET test can be obtained that the porosity of the wood after resin impregnation is almost 0, which indicates that the resin has successfully entered the pores of the wood and the impregnation effect is good.

[0085] The curing behavior of epoxy resin with Schiff base and boron ester bond and double dynamic bond epoxy resin was studied by non-isothermal DSC. As shown in (d) of Figure 4 The three groups of samples all appeared exothermic peaks, and with the extension of heating time, the exothermic peaks also gradually moved to high temperature. The results show that the curing reaction becomes more intense with the increase of temperature. Moreover, there is no obvious post-curing phenomenon, which proves that the samples have been completely cured.

[0086] The glass transition temperature (Tg) of the sample can be observed from the peak temperature of tanδ in the DMA test. The results show that the epoxy resin with added boron ester bonds ( Figure 4 The Tg value of the sample (g) was 45.19℃. The sample with added Schiff base bonds (g) Figure 4 The Tg value of the sample with added double dynamic bonds was 58.28℃. Figure 4 The Tg value of EP (f) is 53.39℃. The upward trend of the Tg value indicates that the double dynamic bond gives EP good heat resistance.

[0087] Under conditions of consistent molecular structure, the glass transition temperature of a resin system is primarily determined by its crosslinking density. Introducing double dynamic bonds into the resin system increases the fluidity of the crosslinked network, thereby lowering the glass transition temperature of the material system. Tg / °C As can be seen from (ef), the glass transition temperature decreases when Schiff bases and boron ester bonds coexist. According to the classical theory of rubber elasticity, the degree of crosslinking of the modified resin system can be calculated using the following formula: Ve = E* / 3RT.

[0088] In the formula: E* is the storage modulus corresponding to the glass transition temperature of the resin system in the 30℃ rubber plateau test; T is the temperature corresponding to 30℃ above Tg in the DMA test, in K, i.e., T=Tg+30; R is the ideal gas constant (8.314J / (mol.K)); Ve is the crosslinking density of the resin system.

[0089] Based on the DMA test data, the crosslinking density of different resin systems was calculated using the above formula, and the results are summarized in Table 1.

[0090] Table 1. Effect of different dynamic bonds on epoxy resin curing

[0091] T / K E / Pa E max / MPa VAP Ve / (mol-m -3 )]]> BDB + VAP 58.28 361.43 1044 9 x 10 6 ]]> 998.36 BDB 53.39 356.54 361.2 5 x 10 6 ]]> 562.25 Figure 5 45.19 348.34 317.5 4 x 10 6 ]]> 460.39

[0092] The results show that the crosslinking density of the epoxy system decreases with the addition of Schiff bases and boronic ester bonds. Although the crosslinking density of the modified resin system decreases significantly with the addition of double dynamic bonds, the storage modulus and glass transition temperature remain at a high level. This is because the structure of the double dynamic bonds is similar to that of thermosetting epoxy resins, generating strong intermolecular forces, which to some extent compensate for the negative impact of the reduced crosslinking density, thus giving the modified resin system better heat resistance.

[0093] Two-dimensional scans were performed in three directions using Micro-CT. For example... Figure 6 As shown, the relatively small pores in the wood further indicate that the impregnation effect is good.

[0094] The three-dimensional structure of the sample was further investigated using micro-CT. For example... Figure 7 As shown, wood impregnated under optimal conditions contains 5.48% wood phase and 38.95% porous phase by volume. This is likely because during long-term high-pressure impregnation, the organic solvent in the resin leaches out the wood cell wall material, causing the cell walls to form macropores or the resin to fill these macropores, thus leading to a decrease in porosity.

[0095] 2.4 Mechanical Properties of Wood Impregnated Dynamically Bonded Modified Epoxy Nanocomposites

[0096] To study the mechanical properties of modified wood, bending performance tests were conducted on the prepared modified wood, such as... Figure 7 As shown in (a). The modulus of rupture (MOR) is an important parameter for evaluating the overall strength of wood; therefore, the higher the modulus of rupture value, the greater the bending strength of the wood. The modulus of elasticity (MOE) represents the wood's ability to resist bending and deformation; therefore, the higher the MOE value, the smaller the deformation of the wood under external force. Initially, the modulus of rupture (MOR) and modulus of elasticity (MOE) of untreated wood were 90 MPa and 8 GPa, respectively. Figure 7 As shown in (cd), the modulus of rupture of dynamically epoxy-modified wood increased from 90 MPa to 156 MPa under impregnation treatment, a significant increase of 73.33% compared to unimpregnated wood. This is mainly due to the full filling of the wood by the dynamic epoxy resin, which strengthens the wood structure.

[0097] In addition, impact strength was measured to assess the toughness of the modified wood, such as... Figure 7 As shown in (f). The results indicate that the IBS of V10-P120-DB-LA has a 30 kJ / m³ content in unmodified wood. -2 Gradually increase to 55 kJ / m -2 This indicates an improvement in toughness. Compared to modification techniques such as heat treatment, furfuryl alcohol treatment, and DMDHEU resin impregnation, which negatively impact wood toughness, this work achieved a simultaneous improvement in both toughness and strength.

[0098] like Figure 8The bending strength of the dynamic epoxy-modified wood was lower (156 MPa) than that of the densified wood reported in the Nature journal (336 MPa), and much lower than that of most alloys. However, it is noteworthy that the MOR and MOE of this modified wood completely exceeded those of other resins (E44 epoxy resin) or chemical reagent-modified (furfuryl alcohol) wood and most materials except diamond. At the same time, wood pretreatment generates a large amount of chemical waste, and the use of a large amount of delignification agent (NaOH and Na2SO3) to prepare composites is also not environmentally friendly. In contrast, the microwave wood pretreatment method has more green and commercial utilization benefits.

[0099] 2.5 Thermal stability and flame retardancy of impregnated wood

[0100] The flame retardancy of pure wood, wood impregnated with different amounts of dynamic boron ester bonding and Schiff base bonding modified epoxy resin, and wood impregnated with the best proportion of dynamic boron ester bonding and Schiff base bonding blended epoxy resin was determined using the UL-94 test.

[0101] The morphology of the samples before and after the UL-94 test is shown in Figure 8 From Figure 8 it can be seen that pure wood failed to pass the UL-94 test because the flame spread rapidly to the top of the sample after one ignition. As shown in Figure 8 , for wood impregnated with a Schiff base, the best flame retardant effect was achieved with 5% VAP impregnation, reaching V-0 level. At the same time, its limiting oxygen index reached 25.2, as shown in Table 2. For boron ester bond impregnated wood, the best flame retardant effect was achieved with 7% BDB impregnated wood, which did not self-ignite after leaving the flame, and had strong self-extinguishing ability, as shown in Figure 8 . For the dynamic boron ester bond, Schiff base bond blended modified wood sample, as shown in Figure 8 , it was ignited after the first contact with the igniter, but immediately extinguished after the sample was moved away from the igniter; the second ignition state was the same, and it was immediately extinguished after leaving the flame. It can be seen that the VAP and BDB introduced by impregnation treatment played a synergistic effect, making the wood have excellent flame retardancy.

[0102] Table 2 LOI and UL-94 test results of dynamic bond impregnated wood

[0103]

[0104] The limiting oxygen index (LOI) test is used to evaluate the minimum oxygen volume fraction required to maintain the continuous combustion of a material. As Figure 9LOI of 5% VAP, 7% BDB and VAP+BDB were 25.2%, 26.5% and 29.3%, respectively, while the LOI of unmodified poplar was only 21%. It is well known that the oxygen content in the atmosphere is about 21%, and the higher the LOI value, the more difficult it is to burn in the atmosphere. The above results demonstrate that the introduction of VAP and BDB can effectively improve the flame retardancy of modified wood.

[0105] Figure 9 Figures (a2) and (a3) show the heat release rate and total heat release rate of wood coated with modified epoxy resin. The total heat release rate and peak heat release rate of pure EP coated wood are high, indicating that its heat insulation performance is poor. The maximum heat release rate and total heat release rate of wood coated with modified epoxy resin are decreased to different degrees. For example, Figure 10 As shown in Figure (a3), the total heat release of wood coated with boron ester bond modified epoxy resin, wood coated with Schiff base modified epoxy resin and wood coated with double dynamic bond modified epoxy resin is 32.5, 21.5 and 17.2 MJ / m 2 The smoke production rate (SPR) of wood coated with double dynamic bond modified epoxy resin is lower than that of wood coated with single dynamic bond modified epoxy resin. This indicates that the mixture of boron ester bond and imine bond has superior smoke suppression and flame retardant performance.

[0106] The fir boards coated with EP, 5% VAP-EP, 7% BDB-EP and VAP+BDB-EP were used for infrared thermal imaging test. The axis is as shown in Figure 10 As shown in Figure (a), the highest temperature in the center of the wood board within 50s of burning time is 304.7°C, 128.2°C, 127.5°C and 118.1°C, respectively.

[0107] Figure 11 Figures (b-e) show the infrared results. The experimental results show that the EP coated fir board burns intensely after 50s, and the temperature on the back of the fir board rises to 304.7°C. The burning temperature of the fir board coated with 5% VAP-EP coating after 50s is 128.2°C, which is reduced by 57.9%. The burning temperature of the fir board coated with 7% BDB-EP coating at 50s is 127.5°C, which is reduced by 58.2%. It is also found that the burning temperature of the fir board coated with VAP+BDB-EP at 50s is only 118.1°C, which is reduced by 61.2%, and VAP and BDB play a synergistic flame retardant effect, thereby improving the flame retardant performance of the wood.

[0108] The residual carbon after the cone calorimeter test was detected by Raman spectroscopy to analyze the influence of boron ester bond and imine bond on the degree of graphitization. The ID / IG value of pure epoxy impregnated wood is 1.08 Figure 11 (a), the ID / IG value of boron ester bond modified epoxy impregnated wood is 0.99Figure 11 In the middle (b)), the ID / IG value of the imine bond modified epoxy impregnated wood is 0.94 Figure 11 In the middle (c)), the ID / IG value of the boron ester bond and imine bond co-modified epoxy impregnated wood is 0.9 Figure 12 In the middle (d)), which shows that the addition of dynamic bond modified epoxy resin enhances the graphitization of the carbon layer.

[0109] 2.6 Flame retardant mechanism

[0110] According to the above results and analysis, the flame retardant mechanism of dynamic bond modified epoxy resin impregnated wood is proposed, as shown in Figure 13 When unmodified poplar burns, lignin, cellulose and hemicellulose in wood react with oxygen, releasing a large amount of heat and smoke, causing intense burning. When wood impregnated with dynamic bond modified epoxy nanocomposites burns, the non-combustible gas produced by thermal decomposition of wood dilutes the oxygen concentration in the air, and the formed phosphoric acid and metaphosphate interfere with and destroy the combustion chain reaction. At the same time, the siloxane groups in POSS play a stabilizing role in the carbonization process, and the dense carbon layer hinders heat transfer and volatilization of pyrolysis products. The combination of nitrogen (N), phosphorus (P), silicon (Si) and boron (B) in the carbon layer can significantly improve the flame retardant effect.

[0111] 2.7 Mechanical properties of compressed wood

[0112] Poplar cubes were impregnated with epoxy resin with the best dynamic bonding rate, then compressed to form compressed wood at 10 megapascals, and the mechanical properties were studied. The results of elastic modulus and breaking modulus show that the mechanical properties of epoxy resin impregnated compressed cubes are better than those of epoxy resin impregnated non-compressed cubes Figure 13 In the middle (a3)). In addition, the multi-layer epoxy impregnated poplar veneer after compression shows stronger comprehensive mechanical properties Figure 14 In the middle (b3)). The improvement of these two mechanical properties is due to the fact that dynamic bonding enhances the interfacial bonding strength during compression-heating, and the slip of dynamic bonding under stress can inhibit the propagation of cracks.

[0113] 2.8 Recyclability and reworkability of epoxy nanomaterials with dynamic bond of Schiff base and boron ester to epoxy resin and impregnated wood

[0114] Dynamic bonds are divided into dynamic non-covalent bonds and dynamic covalent bonds. Dynamic non-covalent bonds include hydrogen bonds, metal ligand bonds, π-π stacking and cation-π interactions, etc. Dynamic covalent bonds include ester bonds, siloxane, imine bonds and disulfide bonds. In recent years, due to the reversibility and breakability of dynamic bonds under certain stimuli, many studies have used the introduction of dynamic bonds in thermosetting polymers to improve the reprocessing, reshaping and recycling ability of thermosetting polymers. In this study, dynamic imine bonds and boron ester bonds were introduced, and the effect of their ratio on reprocessing was studied.

[0115] The recycling of covalently cross-linked thermosetting resins is usually more difficult, but the epoxy resin modified with dynamic imine bonds and boron ester bonds can be easily reprocessed by hot pressing process. As shown in Figure 15 , under high temperature conditions, dynamic imine bonds and boron acid bonds can undergo complex decomposition reactions without using any catalyst, realizing the rearrangement of network topology, so that the network has excellent thermal reprocessing property. Therefore, we use a hot press to explore the reprocessing of epoxy resins with different dynamic bond contents, and test their reprocessing by hot pressing at 100°C for 20min. It is found that the epoxy resin with 7% BDB and 5% VAP has better molding effect.

[0116] As shown in Figure 16 . In order to explore the recyclability of waterborne epoxy resin impregnated wood, the waterborne epoxy resin with the best ratio of double dynamic bonds for flame retardancy, recyclability and water resistance was impregnated into wood, and then the wood was ground into wood powder and pressed into a plate under a hot press at 100°C for 3min. Because boron ester bonds generate a large number of ester groups and hydroxyl groups in the crosslinked network, ester exchange reaction will occur at high temperature, and the reaction of hydroxyl and aldehyde groups in Schiff base bond induces the rearrangement of topological network, so that the epoxy glass material has reshaping property, and therefore the molding property of the pressed plate is better. At the same time, the flame retardant performance of the pressed sheet was preliminarily studied, and it was found that the flame retardant performance was good.

[0117] 2.9 Water absorption and thickness expansion

[0118] After impregnation modification, the water resistance of wood is obviously improved. As shown in Figure 16 , the water absorption of V10-A120 is reduced by 91.5% compared with unmodified wood after impregnation for 288h. With the increase of impregnation time and pressure, the water absorption of V10-A120-DB-LA gradually decreases. The water absorption of V10-P120-DB-LA is further reduced to 15.0%, which is reduced by 105% compared with unmodified wood. The accumulation of resin in the wood prevents water from entering the wood, thereby improving the dimensional stability of the wood.

[0119] Subsequently, the thickness expansion rate of wood is reduced Figure 16The maximum value of thickness swelling rate decreased from 10.2% before modification to 2.3% after modification. In addition, the ASE reached a maximum of 38.4% Figure 16 (d)). This is because the physical barrier formed by in-situ polymerization of the resin (mainly in the wood) reduces the availability of -OH groups in the cell wall of the wood, thereby significantly reducing water absorption. In addition, the modified waterborne epoxy resin can also penetrate the cell wall, further improving the dimensional stability of the modified wood. The addition of the resin reduces the absorption of moisture, which is evidenced by the reduction in EMC Figure 16 (e)). From ​ The improvement of the hydrophobicity of the impregnated wood can be more intuitively seen in (c) and (f).

[0120] 3Conclusion

[0121] In this work, we modified the waterborne epoxy resin by dynamic bonding: boron ester bonding, Schiff base bonding, Fourier transform infrared spectroscopy, XPS, DSC and DMA analysis of the modified epoxy resin, Fourier transform infrared spectroscopy and XPS analysis showed that the flame retardant was successfully synthesized, DSC and DMA analysis showed that the double dynamic bonding made the EP have good heat resistance. The best modified epoxy resin was impregnated into fast-growing wood, and the properties of the impregnated wood after dynamic bonding modification were studied.

[0122] Compared with unmodified impregnated fast-growing wood, the flame retardant, mechanical properties, water resistance and recyclability of the dynamic bonding impregnated wood are greatly improved. In terms of flame retardant performance, compared with unmodified impregnated wood, the LOL value of the double dynamic bonding modified impregnated wood increased by 28.3%, the infrared thermal imaging center temperature decreased by 61.2%, the vertical combustion grade reached V-0, and the combustion time was shortened. In terms of mechanical properties, the elastic modulus of the double dynamic bonding modified impregnated wood increased by 73.33% compared with the unmodified impregnated wood. In terms of waterproof performance, the thickness swelling rate of the wood decreased, and the maximum value decreased from 10.2% before modification to 2.3% after modification. In terms of recyclability, it is divided into the recyclability of modified epoxy resin and the recyclability of impregnated wood, the molding effect of modified epoxy resin is better, and the mechanical properties of the modified epoxy resin impregnated block are improved by 40.7% compared with the unmodified impregnated block.

[0123] This study shows that by introducing dynamic bonds into the epoxy resin to impregnate fast-growing wood, it expands the application range of wood, and has important application prospects in the field of building as a substitute for non-renewable materials.

[0124] The foregoing description of the disclosed embodiments enables a person skilled in the art to make or use the application. Modifications of these embodiments will occur to persons of skill in the art, and that the appended claims are intended to cover all such modifications that do not depart from the true spirit and scope of the application. Therefore, the application is not limited to the embodiments shown but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing a dual dynamic key modified epoxy resin, characterized by, Specifically comprising the following steps: (1) Synthesis of flame retardant VAP containing Schiff base bond Methanol and hydrochloric acid were heated, γ-aminopropyl triethoxysilane was added, and the reaction was carried out at elevated temperature, then cooled, poured into tetrahydrofuran to form white precipitate, washed, vacuum dried to obtain white powder OA-POSS, and then mixed with vanillin in ethanol, and the reaction was carried out at elevated temperature again, and dried to obtain yellow VAP solid powder; (2) Synthesis of flame retardant BDB containing boron ester bond 1,4-benzenediboronic acid and 1-thioglycerol were dissolved in a mixed solvent of tetrahydrofuran and water, magnesium sulfate was added, stirred, filtered and concentrated, and washed to obtain white BDB solid product; (3) Preparation of double dynamic bond modified epoxy resin The epoxy resin, yellow VAP solid powder and white BDB solid product were mixed uniformly to obtain the double dynamic bond modified epoxy resin.

2. The method of claim 1, wherein the modified epoxy resin is a dual dynamic key modified epoxy resin. In step (1), the amount ratio of methanol, hydrochloric acid, γ-aminopropyl triethoxysilane and tetrahydrofuran was 100 mL:15 mL:15.2 mL:200 mL; the amount ratio of OA-POSS, vanillin and ethanol was 2.64 g:3.65 g:50 mL.

3. The method of claim 1, wherein the modified epoxy resin is a dual dynamic key epoxy resin. In step (1), the heating was to 60℃; the reaction was carried out at 90℃ for 18h; the cooling was to room temperature; the washing reagent was anhydrous ethanol; the vacuum drying was at 40℃; the reaction was carried out at 60℃ for 3h again; and the drying was at 40℃.

4. The method of claim 1, wherein the modified epoxy resin is a dual dynamic key modified epoxy resin. In step (2), the amount ratio of 1,4-benzenediboronic acid, 1-thioglycerol, tetrahydrofuran, water and magnesium sulfate was 18.1 mmol:37.1 mmol:80 mL:0.1 mL:5.0 g.

5. The method of claim 1, wherein the modified epoxy resin is a dual dynamic key epoxy resin. In step (2), the stirring was at room temperature for 24h; and the washing reagent was dichloromethane.

6. The method of claim 1, wherein the modified epoxy resin is a dual dynamic key epoxy resin. In step (3), the amount ratio of epoxy resin, yellow VAP solid powder and white BDB solid product was 100 g:5 g:7 g.

7. A double dynamic bond modified epoxy resin prepared by the preparation method of any one of claims 1-6.

8. Application of a double dynamic bond modified epoxy resin prepared by the preparation method of any one of claims 1-6 in wood treatment.

9. A process for the preparation of flame-retardant epoxy-modified wood, characterized in that, Specifically comprising the following steps: (1) Pretreatment of modified wood Fast-growing poplar was soaked in water, then microwave heated after taking out, and freeze-dried to obtain dry poplar; (2) Preparation of flame-retardant epoxy modified wood The dry poplar was added into the double dynamic bond modified epoxy resin prepared by the preparation method of any one of claims 1-6 for vacuum impregnation, pressure maintaining, vacuum sealing heating to obtain the flame-retardant epoxy modified wood.

10. The method for preparing flame-retardant epoxy-modified wood according to claim 9, characterized in that, In step (1), the soaking time is 1 h; the microwave power for the microwave heating is 500 W for 18 min; the density of the dried poplar wood is 0.48-0.52 g / cm 3 ; In step (2), the vacuum impregnation was for 10 min; the pressure maintaining was at 6kPa for 60 min; and the vacuum sealing heating was at 30℃ for 2h.

Images