Preparation method of forest bio-oil photosensitive resin for 3D printing

Abstract

The application discloses a preparation method of a forest bio-oil photosensitive resin for 3D printing, and belongs to the technical fields of 3D printing, non-metal additive and photosensitive resin. The application first prepares modified carbon material and forest bio-oil polyurethane acrylate prepolymer; then the forest bio-oil polyurethane acrylate prepolymer is mixed with an active diluent, and a free radical photoinitiator, a cationic photoinitiator and the modified carbon material are added; vortex and ultrasonic dispersion are conducted to obtain a uniform and stable photosensitive resin. The application synthesizes BPUA by taking forest bio-oil as raw material, and combines the modified carbon material to enhance, so that the integrated bio-based 3D printing material with the characteristics of green renewable, low shrinkage, high strength and high toughness and excellent thermal stability is realized.

Description

Technical Field

[0001] This invention relates to the fields of 3D printing, non-metallic additive manufacturing, and photosensitive resin technology, and more specifically to a method for preparing a photosensitive resin of forest bio-oil for 3D printing. Background Technology

[0002] Currently, fully utilizing forest biomass resources to produce functional resin materials is of great significance. Forest bio-oil obtained through rapid pyrolysis of forest biomass can be used to synthesize bio-oil-based polyurethane acrylate photosensitive resins, possessing advantages such as wide availability, low cost, and green, low-carbon characteristics, making it suitable for photopolymerization 3D printing. However, pure forest bio-oil polyurethane acrylate photosensitive resins suffer from low mechanical strength, insufficient toughness, and high volume shrinkage, making it difficult to meet the requirements for high-precision, high-mechanical-performance printed parts. Carbon nanofibers, graphene, carbon nanotubes, and other nanomaterials possess high modulus and high strength characteristics, and can be used as resin reinforcing fillers. However, unmodified carbon nanomaterials have low surface activity, are prone to agglomeration in photosensitive resins, and have poor dispersibility, leading to increased resin viscosity, decreased photopolymerization efficiency, and poor reinforcing effect. Furthermore, carbon nanomaterials have a shielding effect against ultraviolet light, making it impossible to achieve sufficient curing using conventional printing processes, resulting in warping, deformation, and insufficient strength in printed parts.

[0003] Therefore, providing a method for modifying carbon nanomaterials so that they can be used to enhance photosensitive resins for forest bio-oils and to match 3D printing processes is a problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0004] In view of this, the present invention provides a method for preparing a photosensitive resin of forest bio-oil for 3D printing. The resin prepared by the present invention has the characteristics of suitable viscosity, low volume shrinkage, high mechanical strength, good thermal stability, and high printing accuracy. The present invention also provides a complete preparation method and a matching photopolymerization 3D printing process.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A method for preparing a photosensitive resin of forest tree bio-oil for 3D printing includes the following steps: (1) Preparation of modified carbon materials; (2) Preparation of polyurethane acrylate prepolymers from forest bio-oils (21) Pretreatment of forest bio-oil: The forest bio-oil and ethyl acetate were mixed evenly, allowed to stand and separated into layers. The upper liquid was taken and the ethyl acetate was removed by rotary evaporation to obtain forest bio-oil rich in hydroxyl components. (22) Synthesis of prepolymer of polyurethane acrylate from forest bio-oil: Polyethylene glycol was vacuum dried and then mixed with hydroxyl-rich forest bio-oil (bio-oil substitution rate 20wt%) to obtain a mixture. Hexamethylene diisocyanate and dibutyltin dilaurate catalyst were added. After the reaction was completed, hydroxyethyl 2-methacrylate was added to continue the reaction. After the reaction was completed, the mixture was cooled to room temperature to obtain prepolymer of polyurethane acrylate from forest bio-oil (BPUA). (3) Preparation of polyurethane acrylate photosensitive resin from forest bio-oil: The prepolymer of polyurethane acrylate from forest bio-oil is mixed with an active diluent to obtain a diluted solution. Free radical photoinitiator and cationic photoinitiator are added and mechanically stirred until uniform. Modified carbon material is added, and the mixture is vortexed and then ultrasonically dispersed to obtain a uniform and stable photosensitive resin.

[0007] Furthermore, the modified carbon material mentioned in step (1) is any one of toluene diisocyanate modified graphene oxide, isocyanate modified hydroxylated multi-walled carbon nanotubes, and silane modified carbon nanofibers.

[0008] Furthermore, the preparation method of the silane-modified carbon nanofibers is as follows: 1) Oxidative modification of carbon nanofibers: Carbon nanofibers were added to 65-68wt% concentrated nitric acid, refluxed at 70℃ for 4h, washed with water until neutral, and dried to obtain nitric acid oxidized carbon nanofibers. 2) Nitric acid oxidized carbon nanofibers were dispersed in a mixed solution of anhydrous ethanol and water at a mass ratio of 9:1 and ultrasonically dispersed at 60W and 40KHz for 1h. 1.0wt% of silane coupling agent KH560 was added to the nitric acid oxidized carbon nanofibers, and the mixture was heated and stirred at 70℃ and 800r / min for 12h. After centrifugation, washing, and drying, silane-modified carbon nanofibers (NCNF) were obtained. The mass ratio of silane coupling agent KH560 to the mixed solution is 1:100.

[0009] The beneficial effects of adopting the above-mentioned further scheme are as follows: the present invention uses concentrated nitric acid oxidation and silane coupling agent KH560 graft modification, which can significantly improve the dispersibility of CNF in photosensitive resin and increase the absolute value of Zeta potential by 135.2%.

[0010] Furthermore, the volume ratio of forest bio-oil to ethyl acetate in step (21) is 1:3.

[0011] The settling time is 12 hours.

[0012] Furthermore, the polyethylene glycol was vacuum dried at -0.1 MPa and 40°C for 12 hours. The polyethylene glycol has an average molecular weight of 400, and the mass ratio of polyethylene glycol to hydroxyl-rich forest bio-oil is 80:20. Furthermore, in step (22), the mixture of polyethylene glycol and hydroxyl-rich forest bio-oil is first divided into three parts in a mass ratio of 40:40:20. The first part of the mixture and HDI are poured into the reaction vessel and heated from 25°C to 55°C. The second part of the mixture is then slowly added, followed by the third part of the mixture and the catalyst DBTDA. The mixture is reacted for 4 hours. Finally, the temperature is raised to 60°C, 2-HEMA is added, and the mixture is reacted for 3 hours. The mixture is then cooled to room temperature and discharged.

[0013] The above scheme can effectively regulate the reaction rate and make the reaction more complete.

[0014] Preferably, in step (22), the amount of HDI added is based on an isocyanate index R = 1.05. 2-HEMA was added at an isocyanate index R = 1.05. The mass ratio of DBTDA to the mixture of polyethylene glycol and hydroxyl-rich forest bio-oil is 0.5-1:100.

[0015] More preferably, the amount of forest bio-oil is 40g, the amount of PEG400 is 160g, the amount of HDI is 75.06g, the amount of 2-HEMA is 58.078g, and the amount of dibutyltin dilaurate is 0.1g.

[0016] Furthermore, the active diluent mentioned in step (3) is polyethylene glycol diacrylate; The free radical photoinitiator is diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, and the cationic photoinitiator is diphenyl[4-(phenylthio)phenyl]-sulfuron hexafluoroantimonate.

[0017] Furthermore, the mass ratio of the polyurethane acrylate prepolymer to the reactive diluent is 15:85; The mass ratio of the photoinitiator to the diluent is 2:100, the mass ratio of the cationic photoinitiator to the diluent is 1:100, and the mass ratio of the modified carbon material to the diluent is 0.15:100.

[0018] The beneficial effects of adopting the above-mentioned further scheme are as follows: The present invention uses forest bio-oil to replace polyethylene glycol (PEG) polyol, and reacts it with hexamethylene diisocyanate (HDI) and hydroxyethyl methacrylate (2-HEMA) to prepare bio-oil modified polyurethane acrylate (BPUA) photosensitive prepolymer; it can significantly reduce resin viscosity and volume shrinkage rate, improve material flexibility, tensile strength and leveling properties, and realize green substitution of petrochemical raw materials.

[0019] The optimal addition amount of modified carbon nanofibers (NCNF) is 0.15wt%, which can significantly reduce volume shrinkage and improve tensile strength, elongation at break and hardness while ensuring that the viscosity meets the requirements of photopolymerization printing.

[0020] The present invention also provides a photocuring 3D printing molding method for the photosensitive resin of forest bio-oil prepared by the above method: using an LCD photocuring 3D printer, setting the exposure time to 9s, the printing layer thickness to 0.03mm, the number of bottom layers to 7, and the bottom layer exposure time to 12s, and after printing, curing in an ultraviolet curing chamber for 15min to obtain the molded part.

[0021] The beneficial effects of adopting the above scheme are as follows: the printing parameters of the present invention are set as follows: exposure time of 9s, layer thickness of 0.03mm, and number of bottom layers of 7, the mechanical properties, surface quality and dimensional accuracy of the printed specimens are significantly improved.

[0022] The beneficial effects of this invention are as follows: This invention achieves performance enhancement of photosensitive resins for forest bio-oils through the synergistic effect of carbon nanofiber silane modification, optimal formulation ratio, and proprietary printing process, offering the following advantages compared to existing technologies: 1. Significantly improved dispersibility of carbon nanofibers: After oxidation with concentrated nitric acid and graft modification with KH560, active functional groups are introduced into the surface of carbon nanofibers. The absolute value of the Zeta potential is 135.2% higher than that of unmodified carbon nanofibers. They are uniformly dispersed in the resin without agglomeration, effectively avoiding problems such as sudden increase in resin viscosity and photocuring obstruction caused by agglomeration.

[0023] 2. Optimization of resin rheological and shrinkage properties: The viscosity of the obtained photosensitive resin is 178.2 mPa. It meets the flowability requirements of photopolymer 3D printing; the volume shrinkage rate is 22.43%, which is 33.6% lower than that of pure forest bio-oil polyurethane acrylate resin, significantly reducing warping and deformation of printed parts and improving dimensional accuracy.

[0024] 3. Significantly enhanced mechanical properties: With silane-modified carbon nanofiber reinforcement, the tensile strength of the printed part reaches 16.51 MPa, which is 110.0% higher than that of pure resin; the elongation at break reaches 8.00%, which is 43.5% higher; and the hardness reaches 80.17HD, balancing strength and toughness.

[0025] 4. Improved adaptability of photocuring performance: By optimizing the exposure time and printing layer thickness, the absorption and scattering of ultraviolet light by carbon nanofibers are overcome. The resin transmission depth is 0.43mm and the critical exposure amount is 40.75mJ / cm², which can achieve full curing layer by layer and tight interlayer bonding.

[0026] 5. Improved thermal stability: The initial decomposition temperature of the printed parts reaches 318.79℃, the maximum pyrolysis rate temperature is 424.79℃, and the residual carbon rate is 5.97%. Compared with pure resin, the heat resistance is improved, and it can be used in higher temperature environments.

[0027] 6. Excellent printing quality and molding effect: The molded parts have smooth surfaces and fine structures, and can print hollow, self-supporting and movable structures with no obvious defects and a high molding qualification rate.

[0028] In summary, this invention uses forest bio-oil as raw material to synthesize BPUA and combines it with NCNF reinforcement to achieve an integrated bio-based 3D printing material that is green and renewable, has low shrinkage, high strength and toughness, and excellent thermal stability. Attached Figure Description

[0029] Figure 1 The following are flowcharts and schematic diagrams of the synthesis of BPUA resin prepolymer of the present invention: (a) is a flowchart; (b) is a schematic diagram. Figure 2 This describes the CNF silane modification reaction principle in this invention. Figure 3 The images are SEM images of CNF and NCNF at different magnifications. (a), (b), and (c) are electron microscope images of CNF, and (d), (e), and (f) are electron microscope images of NCNF. Figure 4 Zeta potential analysis plots for NCNF with different amounts of KH560 added; Figure 5 Images of different photosensitive resins and their printed specimens are shown. (a) is the photosensitive resin; (b) is the printed specimen. Figure 6 The figure shows the effect of NCNF on BPUA resin and its printed specimens. (a) Viscosity, (b) Volume shrinkage, (c) Tensile strength, and (d) Hardness. Figure 7 The effect of NCNF addition on BPUA and its printed specimens is shown in the figure. (a) Viscosity, (b) Volume shrinkage, (c) Tensile strength, and (d) Hardness. Figure 8 A curve fitting the exposure intensity and curing depth; Figure 9 The figure shows the effect of exposure time on BPUA / NCNF printed specimens. (a) represents tensile strength, and (b) represents elongation at break. Figure 10 The diagram shows the effect of layer thickness on BPUA / NCNF printed specimens. (a) represents tensile strength, and (b) represents elongation at break. Figure 11 The diagram shows the effect of the printing process on the printed specimen. (a) represents the tensile strength, and (b) represents the elongation at break. Figure 12 For printing the SEM images of the specimen surface, (a) is optimized BPUA, (b) is unoptimized BPUA / NCNF, and (c) is optimized BPUA / NCNF; Figure 13The thermal stability diagrams of the printed samples are shown in (a) and (b) respectively. Figure 14 The following are sample images for printing BPUA / NCNF: (a) school emblem, (b) tree, (c) sunset, (d) hollow sphere, (e) hollow basket, (f) clip, and (g) and (h) clip opening and closing display. Figure 15 For the electron micrographs of the partial structure of the printed sample, (a) is the school emblem, (b) is a tree, (c) is a sunset, (d) is a hollow sphere, (e) is a hollow basket, and (f) is a clip; Figure 16 This is a schematic diagram of the TDI-grafted GO reaction in Example 2; Figure 17 This is a schematic diagram of the reaction principle of TDI grafting and modifying HMWCNTs in Example 3; Figure 18 The following are flowcharts and schematic diagrams of TDI-modified GO and HMWCNTs in Example 3: (a) is a flowchart; (b) is a schematic diagram. Figure 19 The images show the 3D printed samples and partial details of the TDI-GO / BPUA photosensitive resin prepared in Example 2. (a) is a biomimetic horse, (b) is a Taihu stone, (c) is a partial detail of the biomimetic horse, and (d) is a detail of the Taihu stone. Figure 20 The images show the 3D printed samples of TDI-HMWCNTs / BPUA photosensitive resin prepared in Example 3 and their partial details. (a) shows the porous lattice; (b) shows the whale shark; (c) shows the partial details of the porous lattice; and (d) shows the partial details of the whale shark. Detailed Implementation

[0030] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. 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.

[0031] All parts mentioned in the embodiments of this invention are by weight. The names, abbreviations, and specifications of the raw materials used in the embodiments of this invention are shown in Table 1.

[0032] Table 1. Experimental Material Information

[0033] Note that the forest bio-oil used in the examples was prepared from larch by a rapid pyrolysis method. The conditions for rapid pyrolysis were: in a fluidized bed, the pyrolysis temperature was 550°C and the pyrolysis residence time was 2-3 seconds.

[0034] Example 1: A photosensitive resin for 3D printing made from forest bio-oil, the preparation process is as follows: Figure 1 As shown: (1) Add 0.5 parts of carbon nanofibers to 20 parts of 65-68 wt% concentrated nitric acid, reflux at 70℃ for 4 h, wash with water until neutral, and dry to obtain nitric acid oxidized carbon nanofibers; disperse nitric acid oxidized carbon nanofibers in a mixture of 27 parts of anhydrous ethanol and 3 parts of water, ultrasonically disperse for 1 h, add 1.0 wt% of silane coupling agent KH560 of nitric acid oxidized carbon nanofibers, heat at 70℃ and stir at low speed for 12 h, centrifuge, wash and dry to obtain silane modified carbon nanofibers.

[0035] Preparation of modified carbon nanofibers (NCNF): Take 0.5 parts of carbon nanofibers, add 20 parts of concentrated nitric acid, reflux at 70℃ for 4h, wash until neutral and dry; disperse in a mixture of 27 parts of anhydrous ethanol and 3 parts of distilled water, sonicate for 1h, add 1.0wt% silane coupling agent KH560, stir at 70℃ for 12h, centrifuge and dry to obtain NCNF.

[0036] (2) The forest bio-oil and ethyl acetate were poured into a separatory funnel at a volume ratio of 1:3, stirred thoroughly, and allowed to stand for 12 hours. The upper liquid was then removed by rotary evaporation to obtain pretreated forest bio-oil rich in hydroxyl components. PEG400 was vacuum dried for 12 hours and then mixed with the pretreated forest bio-oil (40 g of forest bio-oil and 160 g of PEG400) to obtain a mixture. The mixture was divided into three portions (the mass ratio of the three portions was 40:40:20). The first portion of the mixture and 75.06 g of HDI were poured into a three-necked flask with a condenser. The temperature was raised from 25°C to 55°C, and then the second portion of the mixture was added. Then the third portion of the mixture and 0.1 g of catalyst DBTDA were added, and the reaction was carried out for 4 hours. After the reaction was completed, the temperature was raised to 60°C, and 58.078 g of 2-HEMA was added and reacted for 3 hours. The mixture was then cooled to room temperature and discharged to obtain forest bio-oil polyurethane acrylate prepolymer (BPUA).

[0037] (3) Carbon nanofiber reinforced forest bio-oil photosensitive resin: 15 parts of BPUA prepolymer and 85 parts of PEGDA were mixed, 2 parts of TPO and 1 part of UVI-6976 were added, and the mixture was stirred at 1500 r / min for 1 h; NCNF was added, the mixture was vortexed for 1 min and sonicated for 1 h to obtain the target photosensitive resin. Different photosensitive resins were obtained by adding NCNF to BPUA at amounts of 0.05 parts, 0.10 parts, 0.15 parts, 0.20 parts, and 0.25 parts, respectively.

[0038] Example 2 The scheme is basically the same as that in Example 1, except that the silane-modified carbon nanofibers are replaced with isocyanate-modified graphene oxide.

[0039] The preparation method of isocyanate-grafted modified graphene oxide powder is as follows: 0.3 g of graphene oxide is suspended in 30 mL of DMF (mass-volume ratio of 1:100 g / mL), and ultrasonically treated in an ultrasonic bath for 1.5 h to obtain a GO suspension for later use. Under nitrogen atmosphere, 3 g of TDI and 150 mL of DMF (mass-volume ratio of 1:50 g / mL) are added to a three-necked flask and heated to 80 °C in a water bath. Then, the GO suspension is added dropwise under constant pressure, followed by 4 drops of the catalyst dibutyltin dilaurate (DBTDL). The reaction is carried out under constant temperature and stirring conditions for 8 h. The reactants are centrifuged, the supernatant is discarded, and the remaining product is washed with DMF by vacuum filtration. Then, it is dried in a vacuum oven at 50 °C for more than 12 h to remove the solvent. The dried reactants are then ground in an agate mortar to obtain TDI-functionalized graphene oxide powder, denoted as TDI-GO. Figure 16 The diagram shows the reaction principle of TDI grafting modification of GO. TDI reacts with the hydroxyl / carboxyl groups on the surface of graphene oxide (GO) through isocyanate groups to form covalent urethane bonds between GO sheets.

[0040] Example 3 The scheme is basically the same as that in Example 1, except that the silane-modified carbon nanofibers are replaced with isocyanate-modified hydroxylated multi-walled carbon nanotubes.

[0041] The preparation method of isocyanate-grafted modified hydroxylated multi-walled carbon nanotube powder is as follows: 0.3 g of dried hydroxylated multi-walled carbon nanotubes (HMWCNTs) are added to a centrifuge tube and mixed with 30 mL of N,N-dimethylformamide (DMF) (mass-to-volume ratio of 1:100 g / mL), and ultrasonically treated for 1.5 h. 3 g of TDI and 150 mL of DMF (mass-to-volume ratio of 1:50 g / mL) are placed in a three-necked flask, magnetically stirred, and heated to 55 °C. The uniformly dispersed HMWCNTs suspension is slowly added dropwise to the three-necked flask. After the addition is complete, 4 drops of dibutyltin dilaurate (DBTDL) are added as a catalyst, and the system temperature is raised to 70 °C and the reaction is continuously stirred for more than 8 h to ensure that the reaction proceeds fully. After centrifugation, the supernatant is discarded, and the product is washed by vacuum filtration using DMF. Then, the solid was dried in a vacuum oven at 50°C for more than 12 hours. The dried reactants were then ground in an agate mortar to obtain TDI-functionalized multi-walled carbon nanotube powder, denoted as TDI-HMWCNTs. Figure 17 ).like Figure 18The diagram illustrates the reaction principle of TDI grafting modification of HMWCNTs. The surface of HMWCNTs is rich in hydroxyl groups (-OH), which react with the isocyanate groups (-NCO) in the TDI molecule. During the reaction, the -NCO group of the TDI molecule dehydrates and binds to the -OH group on the HMWCNTs surface, successfully grafting onto the carbon nanotube surface via covalent bonds. The other isocyanate group remains as the active site.

[0042] Comparative Example 1 The scheme is basically the same as that in Example 1, except that no silane-modified carbon nanofibers were added.

[0043] The performance of the products from Examples 1-3 and Comparative Example 1 was compared, and the results are shown in Table 2.

[0044] Table 2

[0045] Comparative Example 2 The product was prepared according to the existing technology CN114292365A, and the results are shown in Table 3.

[0046] Table 3

[0047] Figure 2 This is a diagram illustrating the reaction mechanism between modified carbon nanofibers and KH560 in Example 1. Because CNF itself is chemically inert and has few surface-active groups, concentrated nitric acid was used to oxidize CNF to obtain oxidized carbon nanofibers (OCNF). OCNF contains more reactive hydroxyl groups on its surface, allowing it to react with KH560. Figure 3 The images show scanning electron microscope (SEM) images of CNF and NCNF. As can be seen from the images, the surface of unmodified CNF is relatively smooth, while obvious spherical structures appear on the surface and fiber ends of NCNF, further confirming that CNF reacted with the modifier KH560. Furthermore, the diameter of CNF is approximately 200 nm, and the diameter of NCNF is approximately 250 nm. This is because KH560 reacts with the functional groups on the NCNF surface and forms a coupling agent molecular layer on the NCNF surface, leading to an increase in the diameter of both CNF and NCNF.

[0048] Figure 4The Zeta potentials of 0.5 NCNF, 1.0 NCNF, and 1.5 NCNF prepared with 0.5 wt%, 1.0 wt%, and 1.5 wt% KH560, respectively, were compared, with 1.0 wt% showing the best dispersibility. This is because when the amount of KH560 added is low, it cannot react fully with CNF, resulting in fewer functional groups grafted onto the surface and less steric hindrance between molecules, thus failing to effectively reduce the interaction between fibers. Furthermore, KH560 not only reacts with CNF, but excess KH560 can also undergo self-polymerization, forming a silane coupling agent layer on the CNF surface. The thickening of this silane coupling agent layer leads to a decrease in the interfacial compatibility between CNF and BPUA photosensitive resin. Therefore, 1.0 wt% KH560 is a better modifier addition ratio. Based on the analysis of the oxidant type and modifier addition amount, the optimized modification process was determined to be oxidation of CNF with nitric acid followed by modification with 1.0 wt% KH560.

[0049] Example 4: 3D Printing of Photosensitive Resin for Forest Tree Bio-oil Reinforced with Carbon Nanofiber Reinforcement Photosensitive resin was printed using an LCD 3D printer with a layer thickness of 0.05 mm, 7 base layers, a base layer exposure time of 12 s, and a post-processing time of 15 min. Exposure times were set to 5 s, 7 s, 9 s, and 11 s. To avoid interference with specimen curing, post-processing was omitted. Next, the effect of layer thickness on specimen strength was investigated with layer thicknesses of 0.02 mm, 0.03 mm, 0.04 mm, and 0.05 mm. All other parameters remained the same as the initial process, and no post-processing was performed. The photosensitive resin and specimens showed uniform color, changing from light yellow to black with increasing resin content. Figure 5 ).

[0050] LCD photopolymer printing requires photosensitive resin with good flowability to avoid equipment damage; the resin viscosity should ideally be below 300 mPa·s. Figure 6 (a) The effects of CNF and NCNF on the viscosity of the resin system were compared. Compared with pure BPUA, the viscosity of BPUA / CNF increased by 6.9 mPa·s, which is due to the increased particle content in the system leading to increased viscosity. The viscosity of BPUA / NCNF remained unchanged. This is because surface modification of CNF effectively reduced the affinity and entanglement between particles, and the improved dispersibility helped to reduce the viscosity of the system. Modification treatment can effectively reduce the negative impact of CNF on the viscosity of the resin system.

[0051] Curing shrinkage is caused by the difference in density between liquid resin before and after curing. During the process of monomers being converted into polymers, the interaction between monomer molecules changes from van der Waals forces to covalent bonds, and the structural units are packed more tightly, which leads to volume shrinkage. In severe cases, this can cause warping and deformation of the product, affecting dimensional accuracy and performance. Figure 6 (b) The effects of CNF and NCNF on the volume shrinkage rate of the resin were compared. The volume shrinkage rates of the resins with added CNF and NCNF decreased by 6.6% and 15.1%, respectively. On the one hand, CNF does not shrink during the photocuring process. After the photocuring reaction is completed, CNF can fill the gaps caused by the shrinkage of the forest bio-oil prepolymer and diluent during the curing process, thus reducing the volume shrinkage rate. On the other hand, the surface-modified NCNF has better dispersibility in BPUA photosensitive resin and is easier to disperse evenly in the gaps. Therefore, the surface-modified NCNF can better reduce the volume shrinkage rate of the reinforced photosensitive resin.

[0052] Figure 6 (c) The effects of CNF and NCNF on the tensile strength and elongation at break of the printed specimens were compared. The tensile strength of the printed specimens increased by 11.8% and 21.3% respectively after the addition of CNF and NCNF. This is partly due to the high tensile strength and stiffness of CNF itself, and partly because CNF forms a highly tortuous crack path before fracture, increasing the energy required for complete fiber failure. Therefore, adding CNF helps improve the strength of the composite material. Since the modified NCNF is more uniformly dispersed in BPUA resin and has better interfacial bonding, it is more conducive to the transfer of internal stress in the composite material, thus the strengthening effect of NCNF on tensile strength is more significant. At the same time, the shortened branching increases the interchain spacing and free volume, therefore the toughening effect of adding NCNF on the printed specimens is more significant.

[0053] Figure 6 (d) The effects of CNF and NCNF on the hardness of printed specimens were compared. The hardness of printed specimens was slightly improved after the addition of both CNF and NCNF. This is because CNF has a unique crystal structure that can absorb a large amount of energy and is difficult to be destroyed. Therefore, the addition of CNF can improve the hardness of the composite material. Thanks to the good dispersibility of NCNF, the effect of adding NCNF on improving the hardness of printed specimens is more obvious.

[0054] Compared with unmodified CNF, modified NCNF effectively suppressed the negative impact of CNF addition on resin viscosity, reduced resin volume shrinkage, and improved the tensile strength, toughness, and hardness of printed specimens, confirming that the modification method used in this study is effective and feasible.

[0055] Figure 7(a) The effect of NCNF addition on the viscosity of photosensitive resin was compared. With increasing NCNF addition, the resin viscosity increased. This is because the increased particle concentration in the system reduces the spacing between fibers, making them more prone to entanglement and aggregation, thus increasing the system viscosity. However, the viscosity at all addition levels was less than 300 mPa·s, meeting the requirements for photopolymerization printing. When the NCNF addition was 0.05 wt%, the BPUA / NCNF viscosity remained unchanged; when the addition was 0.25 wt%, the viscosity increased by 22.5%. The negative impact on the resin system viscosity was relatively small.

[0056] Figure 7 (b) The effect of NCNF addition on the volume shrinkage rate of photosensitive resin was compared. With increasing NCNF addition, the volume shrinkage rate of the resin generally showed a trend of first decreasing and then increasing. As mentioned in 4.2.1, adding a certain amount of NCNF helps reduce the volume shrinkage rate of the resin. However, when excessive amounts are added, NCNF fibers become entangled and aggregate, and NCNF particles cannot uniformly fill the voids generated by curing in the BPUA resin, thus weakening the inhibitory effect on curing shrinkage. The volume shrinkage rate was lowest when the NCNF addition was 0.15%, a reduction of 33.6% compared to pure BPUA, demonstrating a significant effect.

[0057] Figure 7 (c) The effects of NCNF addition on the tensile strength and elongation at break of the printed specimens were compared. With increasing NCNF addition, the tensile strength and elongation at break of the printed specimens generally showed a trend of first increasing and then decreasing. This is because when the fiber concentration in the system is too high, entanglement and agglomeration occur between the fibers, resulting in poor uniformity of the composite material. Under stress, the internal stress distribution is uneven, making it more susceptible to failure. Furthermore, agglomerated NCNF also blocks ultraviolet light, interfering with the curing of the photosensitive resin, leading to a decrease in the printing quality of the specimens and adversely affecting their strength. The tensile strength and elongation at break were highest when the NCNF addition was 0.15%, with tensile strength increasing by 34.8% and elongation at break increasing by 41.1%. The tensile strength (11.71-16.51 MPa) and elongation at break (4.23-8.00%) of the BPUA / NCNF specimens were similar to those of the lignin-acrylic photosensitive resin prepared by Sutton et al. (11-15 MPa and 3.0-7.6%).

[0058] Figure 7(d) The effect of NCNF addition on the hardness of printed specimens was compared. With increasing NCNF addition, the hardness of the printed specimens generally showed a trend of first increasing and then decreasing, but the overall fluctuation was small. The author believes this is because, on the one hand, the hardness-enhancing effect of NCNF mainly benefits from its unique crystal structure, and increasing the NCNF addition in the resin system has no significant impact on its crystal structure. On the other hand, as the NCNF concentration in the resin system increases, the uniformity of the composite material decreases. This difference is more pronounced in the length direction (gauge length of the tensile specimen is 60 mm), while the difference in the thickness direction is smaller (the thickness of the hardness test specimen is only 4 mm). Therefore, the change in hardness with addition is not as significant as the change in tensile strength. When the NCNF addition is 0.15 wt%, the maximum hardness reaches 80.17 HD.

[0059] The optimal addition amount of NCNF to reinforce BPUA photosensitive resin is 0.15 wt%. At this ratio, the viscosity of BPUA / NCNF increases by only 9.4%, the volume shrinkage decreases by 33.6%, the tensile strength increases by 34.8%, the elongation at break increases by 41.1%, and the hardness increases by 5.9%. Therefore, adding 0.15 wt% NCNF to BPUA is the optimal ratio for reinforcing photosensitive resin.

[0060] Because black NCNF blocks and shields ultraviolet light, it interferes with the photocuring process of the photosensitive resin. Theoretically, BPUA / NCNF requires longer exposure times and energy to achieve full curing. The depth of penetration (Dp) and critical exposure intensity (Ec) are crucial indicators reflecting the photocuring performance of the photosensitive resin and determining the corresponding printing parameters. The layer thickness of the printed sample must be less than the depth of penetration (DP) of the slurry for continuous adhesion between layers; the exposure intensity (E) must be greater than the critical exposure intensity (Ec) for a curing reaction to occur.

[0061] Using a UV-LED curing chamber, with a light power density of 90 mW / cm², BPUA and BPUA / 0.15NCNF were exposed for 2s, 3s, 4s, 5s, and 6s to achieve different exposure intensities E, and the curing depth Cd of the resin was measured. The curing curve of the photosensitive resin, Cd = DplnE - DplnEc, can be fitted using E and Cd. Ec and Dp can then be calculated. The curing curves and parameters of BPUA / NCNF are shown in Table 4. Figure 8 As shown.

[0062] The Dp of BPUA / NCNF is approximately 0.43 mm, which is 26.88% lower than that of BPUA, and the Ec is approximately 40.75 mJ / cm. 2The concentration of NCNF increased by 9.14% compared to BPUA. This is mainly because black NCNF absorbs, refracts, and scatters ultraviolet light, hindering its penetration into the resin. It also causes uneven illumination of the BPUA / NCNF photosensitive resin, thus reducing the penetration depth of ultraviolet light into the resin and increasing the energy required for curing. Although the addition of NCNF has a certain negative impact on the photocurability of BPUA photosensitive resin...

[0063] Based on the above analysis, BPUA / NCNF exhibits good photocuring performance. Since the transmission depth Dp of BPUA / NCNF photosensitive resin is smaller than that of pure BPUA resin, the required printing layer thickness should be appropriately reduced to ensure sufficient UV light penetration and complete resin curing; the printing layer thickness should be less than 0.43 mm. Because the critical exposure dose Ec of BPUA / NCNF photosensitive resin is larger than that of pure BPUA resin, a higher exposure intensity is required. Since the power of the photocuring printer used in this study is fixed, it is necessary to increase the exposure intensity by extending the printing exposure time to promote complete curing of the BPUA / NCNF photosensitive resin.

[0064] Table 4. Parameters of the fitting curves for exposure intensity and curing depth

[0065] Figure 9 The effects of different exposure times on the tensile strength and elongation at break of printed specimens were compared. Figure 9 (a) It can be seen that the tensile strength first increases and then tends to stabilize as the exposure time increases. This is because with the extension of exposure time and the increase of light intensity, the BPUA / NCNF photosensitive resin is cured more completely, the degree of polymerization between molecules increases, and a cross-linked network structure is formed, making the printed specimen more dense and thus increasing the tensile strength. However, since the molecular weight of the resin is fixed, its degree of polymerization is also limited. When the polymerization reaches a certain degree, due to the reduction of reaction sites and the increase of steric hindrance, the polymerization reaction is difficult to continue, the degree of cross-linking no longer increases, and therefore the tensile strength no longer increases. Figure 9 (b) It can be seen that the elongation at break first increases and then decreases. This is because the increased exposure time intensifies cross-linking, resulting in tighter intermolecular bonding, making the specimen less prone to deformation and breakage under stress, and enhancing toughness. However, on the other hand, as the degree of curing continues to increase, the mobility of molecular chain segments also decreases, and the ductility of the material decreases. Therefore, excessively long exposure time will actually decrease the elongation at break. Compared with the initial process (exposure time 5s), when the exposure time is 9s, the tensile strength of the specimen increases by 25.8%, and the elongation at break increases by 20.6%.

[0066] Figure 10 The effects of different layer thicknesses on the tensile strength and elongation at break of printed specimens were compared. Figure 10 As can be seen from (a) and (b), the tensile strength and elongation at break of the specimens first increase and then decrease as the layer thickness decreases. This is because during the printing process, the liquid surface of the resin closer to the screen receives greater light intensity, while the liquid surface farther from the screen receives weaker light intensity, resulting in differences in the degree of curing in the thickness direction and uneven material properties. This leads to uneven internal stress distribution under stress, making it prone to failure. The smaller the printed layer thickness, the more uniform the degree of curing in the thickness direction, and the more uniform the stress distribution under stress, thus improving tensile strength and toughness. However, when the layer thickness is 0.02 mm, the tensile strength and toughness of the specimens decrease significantly. This may be due to the LCD printing principle, where the printing platform rises one layer height after each layer is printed, and the remaining resin in the liquid tank flows and fills the gap between the platform and the screen before the next layer is printed. When the distance between the printing platform and the screen is only 0.02mm, the resin, which has a certain viscosity, has difficulty flowing in the narrow gap. If the resin cannot fully level and fill the gap between the platform and the screen during the time interval between two exposures, defects such as voids, bubbles, or uneven thickness will appear in the printed layer, leading to a decrease in the quality of the printed specimen and making it prone to failure under stress at the defective locations. Compared with the initial process (layer thickness 0.05mm), when the layer thickness is 0.03mm, the tensile strength increases by 20.1%, and the elongation at break increases by 30.3%.

[0067] Figure 11 (a) and (b) compare the tensile strength and elongation at break of BPUA and BPUA / NCNF printed specimens using an initial process suitable for BPUA photosensitive resin (exposure time 5s, layer thickness 0.05mm), and optimized BPUA / NCNF printed specimens using optimized process parameters. The optimized BPUA / NCNF printed specimens showed a 61.4% increase in tensile strength and a 29.5% increase in elongation at break compared to the unoptimized specimens. The synergistic effect of reducing the printing layer thickness and increasing the exposure intensity was more significant than that of optimizing a single parameter. Compared to BPUA printed specimens, the optimized BPUA / NCNF specimens showed a 110.0% increase in tensile strength and a 43.5% increase in elongation at break, indicating that selecting a suitable printing process is crucial to maximizing the reinforcing effect of NCNF.

[0068] Figure 12The images show the microscopic surface morphology of optimized BPUA / NCNF printed specimens, unoptimized BPUA / NCNF printed specimens, and optimized BPUA / NCNF printed specimens, captured using scanning electron microscopy. It can be seen that the optimized BPUA / NCNF printed specimen has a smoother surface and better print quality compared to the unoptimized and optimized BPUA / NCNF specimens. This is because the initial printing process is more suitable for BPUA photosensitive resin and cannot fully cure the BPUA / NCNF photosensitive resin, which requires higher exposure intensity. Therefore, the unoptimized specimen has more surface defects, while the optimized process allows the BPUA / NCNF resin to cure more fully, improving surface quality.

[0069] Table 5 shows the pyrolysis parameters of BPUA-printed specimens, unoptimized BPUA / NCNF-printed specimens, and optimized BPUA / NCNF-printed specimens. Figure 13 (a) and (b) are the TG and DTG curves of the three printed specimens, respectively. The thermal stability relationship of the three photosensitive resins is: optimized BPUA / NCNF > BPUA > unoptimized BPUA / NCNF.

[0070] Table 5. Pyrolysis parameters of printed specimens

[0071] Compared to BPUA-printed specimens, the optimized BPUA / NCNF-printed specimens exhibited better thermal stability, indicating that the addition of NCNF contributes to improving the thermal stability of the printed specimens. This is due, on one hand, to the inherent good thermal stability of NCNF, which, as a filler, can enhance the thermal stability of the composite material; on the other hand, the functional groups on the NCNF surface can also participate in the photopolymerization process, forming covalent bonds with the forest bio-oil prepolymer and diluent, increasing the degree of crosslinking of the photosensitive resin and delaying its thermal decomposition.

[0072] However, compared to the other two types of printed specimens, the unoptimized BPUA / NCNF specimen exhibited poorer thermal stability. This is because the specimen did not cure completely during the printing process, resulting in a lower degree of resin cross-linking. The incompletely cured diluent and other small molecules were more prone to pyrolysis, leading to earlier pyrolysis and a lower residual carbon content. Furthermore, the DTG curve of the unoptimized BPUA / NCNF fluctuated between 250-350℃, with the pyrolysis rate initially increasing, then slowing down, and then increasing again. This may be because the prepolymer and diluent, which did not bind with NCNF, underwent pyrolysis first, while the pyrolysis of molecules covalently bonded to NCNF was suppressed.

[0073] In summary, a suitable printing process is required to fully realize the effect of NCNF in improving the thermal stability of the specimens. The optimized BPUA / NCNF exhibited better thermal stability below 318℃. Compared with the thermal stability of the photosensitive resin with added acrylic epoxidized soybean oil prepared by Rosa et al. (T5% 272~290℃, maximum loss rate temperature 340~397℃, carbon residue 0.7~1.0%), the photosensitive resin for reinforcing forest bio-oil prepared in this study has better thermal stability.

[0074] Example 5: Demonstration of NCNF-reinforced forest bio-oil photosensitive resin printed structures Figure 14 This exhibition showcases a variety of structural samples printed using an optimized printing process with NCNF-reinforced photosensitive resin containing bio-oil from forest trees in the best possible ratio. Figure 15 The image shows detailed images of the sample under an electron microscope. BPUA / NCNF resin can print not only planar samples, but also intricately sculpted samples, curved structures, and flexible samples.

[0075] The lettering on the school emblem sample is clear, and the tree and sunset patterns are sharp and smooth. The printed hollow sphere requires no support and is self-supporting, with a very fine network structure. The basket sample consists of several curved lines, with a fine and sturdy structure; it can withstand the weight of a beaker containing 80g of distilled water without collapsing. The printed self-assembly clips can be assembled seamlessly, indicating that the printed specimens have good dimensional accuracy, thanks to the good toughness and flexibility of BPUA / NCNF resin. The self-assembly clips can also be repeatedly opened and closed by the extension and compression of the wave-shaped structural components. Figure 15 ).

[0076] BPUA / NCNF photosensitive resin uses bio-oil derived from forests as raw material and modified carbon nanofibers as reinforcing material. The addition of NCNF enhances the tensile strength, toughness, dimensional accuracy, and thermal stability of the printed specimens. The optimized printing process improves the surface quality, specimen strength, and thermal stability. This material is expected to be used for printing flexible devices, curved parts, heat-resistant parts, and complex and delicate jewelry and handicrafts.

[0077] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for preparing a photosensitive resin of forest bio-oil for 3D printing, characterized in that, Includes the following steps: (1) Preparation of modified carbon materials; The modified carbon material is silane-modified carbon nanofiber; The preparation method of the silane-modified carbon nanofibers is as follows: 1) Oxidative modification of carbon nanofibers: Carbon nanofibers were added to 65-68wt% concentrated nitric acid, refluxed at 70℃ for 4h, washed with water until neutral, and dried to obtain nitric acid oxidized carbon nanofibers. 2) Silane grafting modification: Nitric acid oxidized carbon nanofibers were dispersed in a mixed solution of anhydrous ethanol and water at a mass ratio of 9:1 and ultrasonically dispersed at 60W and 40KHz for 1h. 1.0wt% of silane coupling agent KH560 was added to the nitric acid oxidized carbon nanofibers, and the mixture was heated at 70℃ and 800r / min with low-speed stirring for 12h. After centrifugation, washing, and drying, silane-modified carbon nanofibers were obtained; the mass ratio of silane coupling agent KH560 to the mixed solution was 1:

100. (2) Preparation of polyurethane acrylate prepolymers from forest bio-oils; (21) Pretreatment of forest bio-oil: The forest bio-oil and ethyl acetate were mixed evenly, allowed to stand and separated into layers. The upper liquid was taken and the ethyl acetate was removed by rotary evaporation to obtain forest bio-oil rich in hydroxyl components. (22) Synthesis of polyurethane acrylate prepolymer of forest bio-oil: Polyethylene glycol was vacuum dried and then mixed with hydroxyl-rich forest bio-oil to obtain a mixture. Hexamethylene diisocyanate and dibutyltin dilaurate were added in three batches. After the reaction was completed, hydroxyethyl 2-methacrylate was added to continue the reaction. After the reaction was completed, the mixture was cooled to room temperature to obtain polyurethane acrylate prepolymer of forest bio-oil. (3) Preparation of polyurethane acrylate photosensitive resin from forest bio-oil: The prepolymer of polyurethane acrylate from forest bio-oil is mixed with an active diluent to obtain a diluted solution. Free radical photoinitiator and cationic photoinitiator are added, and the mixture is mechanically stirred until uniform. Modified carbon material is added, and the mixture is vortexed and then ultrasonically dispersed to obtain a uniform and stable photosensitive resin. The reactive diluent is polyethylene glycol diacrylate; the free radical photoinitiator is diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide; the cationic photoinitiator is diphenyl[4-(phenylthio)phenyl]-sulfonium hexafluoroantimonate; the mass ratio of the polyurethane acrylate prepolymer to the reactive diluent is 15:85; the mass ratio of the photoinitiator to the diluent is 2:100; the mass ratio of the cationic photoinitiator to the diluent is 1:100; and the mass ratio of the modified carbon material to the diluent is 0.15:

100.

2. The method for preparing a photosensitive resin of forest bio-oil for 3D printing according to claim 1, characterized in that, The volume ratio of forest bio-oil to ethyl acetate in step (21) is 1:3; The settling time is 12 hours.

3. The method for preparing a photosensitive resin of forest bio-oil for 3D printing according to claim 1, characterized in that, In step (22), the polyethylene glycol is vacuum dried at -0.1 MPa and 40°C for 12 hours. The average molecular weight of the polyethylene glycol is 400, and the mass ratio of polyethylene glycol to hydroxyl-rich forest bio-oil is 80:

20.

4. The method for preparing a photosensitive resin of forest bio-oil for 3D printing according to claim 3, characterized in that, In step (22), the mixture of polyethylene glycol and hydroxyl-rich forest bio-oil is first divided into three parts with a mass ratio of 40:40:

20. The first part of the mixture and hexamethylene diisocyanate are poured into the reaction vessel and heated from 25°C to 55°C. Then the second part of the mixture is added, followed by the third part of the mixture, the catalyst, and dibutyltin dilaurate. The mixture is reacted for 4 hours. Finally, the temperature is raised to 60°C, hydroxyethyl 2-methacrylate is added, and the mixture is reacted for 3 hours. The mixture is then cooled to room temperature and discharged.

5. The method for preparing a photosensitive resin of forest bio-oil for 3D printing according to claim 1, characterized in that, It also includes step (4) using the prepared forest bio-oil photosensitive resin for 3D printing.

6. The method for preparing a photosensitive resin of forest bio-oil for 3D printing according to claim 5, characterized in that, The photopolymerization molding method for 3D printing is as follows: using an LCD photopolymerization 3D printer, setting the exposure time to 9s, the printing layer thickness to 0.03mm, the number of bottom layers to 7, and the bottom layer exposure time to 12s, and then curing it in an ultraviolet curing chamber for 15min after printing to obtain the molded part.

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