Method for preparing gamma ray protection aramid paper by using waste aramid 1313

Abstract

The application provides a method for preparing a gamma-ray protection aramid paper by using waste aramid 1313, and belongs to the technical field of radiation protection material preparation. The waste aramid 1313 is pretreated and fibrillated and cut to obtain regenerated aramid 1313 short fiber slurry; bismuth tungstate powder is prepared by a hydrothermal reaction and mixed with metal gadolinium powder to obtain a composite shielding powder; the composite shielding powder is added into water-based polyurethane emulsion to be modified to obtain modified composite shielding powder; the regenerated aramid 1313 short fiber slurry, aramid fibrid fiber and the modified composite shielding powder are mixed to obtain a protection aramid paper pulp, and the product is obtained through wet papermaking, dehydration, pre-drying and hot pressing and setting. The application turns waste into treasure, utilizes the fibrillated structure of the regenerated fiber and the synergistic crosslinking effect of the water-based polyurethane to enhance the powder combination firmness, utilizes the complementary elimination of the weak radiation absorption area of multiple elements to eliminate the weak radiation absorption area, and improves the overall shielding performance and folding resistance of the aramid paper.

Description

Technical Field

[0001] This invention belongs to the field of radiation protection material preparation technology, specifically relating to a method for preparing gamma-ray protective aramid paper using waste aramid 1313. Background Technology

[0002] The current mainstream gamma-ray shielding aramid paper preparation process mainly uses virgin aramid 1313 fiber as the main skeleton, and uses high-temperature calcination to prepare inorganic shielding powder. The inorganic shielding powder is then mixed with aramid pulp and wet papermaking is carried out.

[0003] In actual production, virgin aramid 1313 is expensive, while large quantities of waste aramid 1313 are generally considered to have compromised mechanical strength, making it difficult to utilize as the main skeleton of specialty paper for high-value purposes. In addition, when inorganic shielding powder is mixed with aramid fibers, it usually causes the powder to agglomerate within the paper base, and the inorganic shielding powder frequently falls off during subsequent folding and service. Summary of the Invention

[0004] This invention overcomes the shortcomings of the prior art and provides a method for preparing gamma-ray protective aramid paper using waste aramid 1313.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows: a method for preparing gamma-ray shielding aramid paper using waste aramid 1313, comprising the following steps:

[0006] S1: Pre-treat and fibrillate waste aramid 1313 to prepare recycled aramid 1313 chopped fiber slurry.

[0007] S2: Bismuth tungstate powder is prepared by hydrothermal reaction. The bismuth tungstate powder is mixed and ground with gadolinium metal powder to obtain composite shielding powder.

[0008] S3: The composite shielding powder is added to an aqueous polyurethane emulsion, and the interface is modified by dispersion and stirring. After drying and grinding, the modified composite shielding powder is obtained.

[0009] S4: Mix recycled aramid 1313 chopped fiber pulp with aramid precipitated fiber to prepare a mixed pulp, add modified composite shielding powder and stir to disperse to obtain protective aramid pulp;

[0010] S5: The protective aramid pulp is wet-processed, dehydrated, and pre-dried to form a wet paper web, which is then hot-pressed and cooled to obtain γ-ray protective aramid paper.

[0011] Preferably, step S1 specifically includes:

[0012] Waste aramid 1313 was cut into blocks and then subjected to ultrasonic cleaning with water, degreasing cleaning with alkaline solution, and rinsing with pure water in sequence. After drying, the pretreated waste aramid 1313 was obtained.

[0013] The pretreated waste aramid 1313 was fed into a two-stage shearing machine for multi-stage shearing, and then mechanically opened using a comb-type opening machine to induce fibrillation of the fibers.

[0014] A double-layer vibrating screener was used for screening to collect fibrillated aramid 1313 short fibers with a length of 2.0 mm to 3.5 mm.

[0015] The fibrillated aramid 1313 chopped fibers were mixed with pure water to form a fiber suspension, which was then dissociated at high speed to obtain a recycled aramid 1313 chopped fiber slurry.

[0016] Preferably, the alkaline solution degreasing and cleaning uses a sodium carbonate aqueous solution or sodium bicarbonate aqueous solution with a mass fraction of 2% to 5%, at a temperature of 40°C to 50°C, for a duration of 20 to 30 minutes.

[0017] The drying process continues until the moisture content of the waste aramid 1313 is ≤3%;

[0018] In multi-stage shearing, the tool gap is controlled to be 5mm to 8mm in the first-stage shearing process and 2mm to 3mm in the second-stage shearing process.

[0019] The fiber suspension has a mass concentration of 1% to 2%, and the high-speed dissociation dispersion speed is 12,000 r / min to 15,000 r / min, with a duration of 15 min to 30 min.

[0020] Preferably, in step S2, the process of preparing bismuth tungstate powder by hydrothermal reaction includes:

[0021] Bismuth nitrate is dissolved in dilute nitric acid to obtain an acidic bismuth nitrate solution, and then an aqueous sodium tungstate solution is added to obtain a mixed salt solution.

[0022] Adjust the pH of the mixed salt solution to 5.0 to 5.5, add hexadecyltrimethylammonium bromide and polyethylene glycol to obtain a bismuth tungstate precursor solution;

[0023] The bismuth tungstate precursor solution was placed in a closed hydrothermal reactor and subjected to a hydrothermal reaction at 160°C to 200°C for 12 to 24 hours.

[0024] After cooling, the product was removed and subjected to centrifugation, water washing, anhydrous ethanol washing, and drying to obtain bismuth tungstate powder.

[0025] Preferably, in step S2, the particle size of the gadolinium metal powder is 50 nm to 200 nm, and the mass ratio of bismuth tungstate powder to gadolinium metal powder is 3:1 to 6:1.

[0026] Preferably, in step S3, the waterborne polyurethane emulsion is a waterborne polyurethane elastomer emulsion with a heat resistance temperature ≥220℃, a solid content of 20% to 40%, and an emulsion particle size of 50nm to 200nm; the mass ratio of the composite shielding powder to the solids in the waterborne polyurethane emulsion is 2:1 to 5:1.

[0027] Preferably, the interface modification process in step S3 specifically includes:

[0028] The composite shielding powder was added to an aqueous polyurethane emulsion and subjected to ultrasonic dispersion for 20 to 30 minutes.

[0029] Then, mechanical stirring was carried out for 2 to 3 hours at a speed of 400 to 600 r / min.

[0030] The mixed system was dried in an environment of 60°C to 70°C and then ground to obtain modified composite shielding powder.

[0031] Preferably, in step S4:

[0032] The mass ratio of the dry weight of recycled aramid 1313 chopped fiber to the dry weight of aramid precipitated fiber in the slurry is 5:5 to 6:4.

[0033] The mass concentration of the mixed slurry is 0.05% to 0.1%;

[0034] The ratio of the dry weight of the modified composite shielding powder to the total dry weight of the aramid fibers in the mixed slurry is 1:3 to 1:5.

[0035] During the process of adding the modified composite shielding powder and stirring and dispersing it, the mechanical stirring speed is controlled at 800 r / min to 1200 r / min, and the time is 1 h to 2 h.

[0036] Preferably, in step S5:

[0037] The wet papermaking dehydration process includes: wet papermaking using a 200-mesh filter, followed by vacuum dehydration for 5 to 10 minutes under an ambient pressure of -0.08 MPa to -0.1 MPa;

[0038] The internal temperature is controlled at 75℃ to 85℃ during the pre-drying process, and the pre-drying time is 0.5h to 1h.

[0039] Preferably, in step S5, the parameters for hot pressing include: hot pressing temperature of 260°C to 280°C, applied pressure of 14MPa to 20MPa, and holding pressure for 2 minutes to 8 minutes.

[0040] This invention, through specific treatment of waste aramid 1313, compounding of shielding elements, and interface modification combined with hot pressing, effectively improves the bonding strength and radiation attenuation efficiency of modified composite shielding powder in aramid paper matrix, achieving significant technological progress and possessing the following beneficial effects:

[0041] This invention uses recycled aramid 1313 chopped fiber slurry, prepared through pretreatment and fibrillation, as the main skeleton material. Under multi-stage shearing and mechanical opening, the orientation of the macromolecular chains on the surface of the recycled aramid 1313 chopped fiber slurry is forcibly disrupted, transforming the surface from a dense, smooth state to a rough, porous, and barbed fibrillated morphology. This fibrillation provides abundant contact sites for the modified composite shielding powder, which can be stably embedded in the micropores on the surface of the recycled aramid 1313 chopped fiber. Compared to existing technologies that use dense, smooth virgin aramid 1313 fibers, making it difficult for inorganic powders to adhere, this invention utilizes the service damage and fibrillation treatment of waste aramid 1313 to enhance the mechanical anchoring effect of the aramid fiber skeleton on the inorganic powder, resulting in a more stable aramid paper-based structure.

[0042] This invention employs a composite shielding powder, prepared by hydrothermal reaction of bismuth tungstate powder and ground with gadolinium powder, as the radiation functional phase. The composite shielding powder contains bismuth, tungsten, and gadolinium, each with different K-layer characteristic absorption edges. The photoelectric effect and Compton scattering high absorption range corresponding to bismuth, tungsten, and gadolinium form a continuous coverage. When incident gamma rays interact with the composite shielding powder, the powder exhibits graded absorption and attenuation of the incident gamma rays across different photon energy ranges. Compared to existing technologies using single-component inorganic powders with characteristic weak absorption regions, this invention eliminates the weak absorption regions of single elements through complementary characteristic absorption edges of multiple high atomic number elements, thereby improving the overall shielding performance of gamma-ray protective aramid paper against broad-spectrum gamma rays.

[0043] This invention combines the interface modification process of waterborne polyurethane elastomer emulsion on composite shielding powder with the fibrillation structure of recycled aramid 1313 chopped fiber slurry and the hot pressing process. The hot pressing temperature is set close to the glass transition temperature of aramid 1313. The waterborne polyurethane elastomer emulsion coating the surface of the modified composite shielding powder undergoes thermoplastic melting. Under the applied pressure, the viscous waterborne polyurethane material deeply penetrates into the microfibrillated pores and barbed seams on the surface of the recycled aramid 1313 chopped fibers. Simultaneously, the polar urethane groups in the molecular chain of the waterborne polyurethane material undergo intermolecular hydrogen bond rearrangement with the amide groups on the surface of the recycled aramid 1313 chopped fibers. The deep synergy between this mechanical anchoring effect and the intermolecular hydrogen bonding effect constructs a continuous cross-linked network inside the γ-ray protective aramid paper, producing a significant effect that surpasses single physical adsorption. This overcomes the defect in existing technologies where inorganic powders are easily shed in large quantities during repeated folding and service, providing the γ-ray protective aramid paper with superior folding resistance and powder shedding performance. Attached Figure Description

[0044] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0045] Figure 1 This is a schematic diagram of a process for preparing gamma-ray shielding aramid paper using waste aramid 1313;

[0046] Figure 2 This is a schematic diagram illustrating the principle of the synergistic effect of gamma-ray gradient absorption. Detailed Implementation

[0047] 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.

[0048] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein. Therefore, the scope of protection of the invention is not limited to the specific embodiments disclosed below.

[0049] In the description of this application, it should be understood that the terms "center," "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicating orientation or positional relationships based on the orientation or positional relationships shown in the accompanying drawings, are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as limiting the scope of protection of this application. Furthermore, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, features defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0050] In the description of this application, it should be noted that, unless otherwise expressly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections or electrical connections; they can refer to direct connections or indirect connections through an intermediate medium; and they can refer to the internal communication between two components. Those skilled in the art will understand the specific meaning of the above terms in this application based on the specific circumstances.

[0051] Unless otherwise specified, all raw materials described below may be commercially available or prepared using conventional methods in the art. Exemplary commercially available specifications are as follows: hexadecyltrimethylammonium bromide (CTAB, purity ≥99%), sodium tungstate (purity ≥99.5%), polyethylene glycol (PEG-400), bismuth nitrate (purity ≥99.0%), gadolinium metal powder (purity ≥99.9%, customized particle size 50nm-200nm), aqueous polyurethane elastomer emulsion (cationic, solid content 30%, viscosity ≤200mPa·s), aramid precipitated fiber (Taipron®, precipitated pulp), virgin aramid 1313 (Taipron®, chopped fiber), and waste aramid 1313 (labor protection waste, obtained through recycling).

[0052] Application Overview:

[0053] The inherent contradiction between the easy detachment of inorganic shielding powder and its limited shielding performance lies in the extremely high crystallinity and dense, smooth surface of virgin aramid 1313 fibers, which are chemically inert. This near-frictionless surface morphology is the physical reason why inorganic shielding powder is difficult to adhere to and easily detaches. Simultaneously, different elements possess fixed K-layer characteristic absorption edges, and the absorption spectrum of a single element cannot cover complex broad-energy radiation, inevitably resulting in weak absorption regions within specific energy ranges. Based on this analysis, this invention constructs a high-strength interfacial bonding network between the composite shielding powder and recycled aramid 1313 chopped fibers to prevent the composite shielding powder from detaching, while simultaneously broadening the synergistic absorption range of inorganic shielding powder for γ-rays of different energy bands.

[0054] Exemplary method:

[0055] like Figure 1 As shown, a method for preparing gamma-ray shielding aramid paper using waste aramid 1313 includes the following steps:

[0056] S1: Pre-treat and fibrillate waste aramid 1313 to prepare recycled aramid 1313 chopped fiber slurry.

[0057] S2: Bismuth tungstate powder is prepared by hydrothermal reaction. The bismuth tungstate powder is mixed and ground with gadolinium metal powder to obtain composite shielding powder.

[0058] S3: The composite shielding powder is added to an aqueous polyurethane emulsion, and the interface is modified by dispersion and stirring. After drying and grinding, the modified composite shielding powder is obtained.

[0059] S4: Mix recycled aramid 1313 chopped fiber pulp with precipitated fiber to prepare a mixed pulp, add modified composite shielding powder and stir to disperse to obtain protective aramid pulp;

[0060] S5: The protective aramid pulp is wet-processed, dehydrated, and pre-dried to form a wet paper web, which is then hot-pressed and cooled to obtain γ-ray protective aramid paper.

[0061] Below, each step will be explained in detail based on the design principles.

[0062] In the current process of preparing lead-free shielding aramid paper, virgin aramid 1313 fibers are used as the matrix skeleton. Virgin aramid 1313 fibers have high crystallinity, a dense and smooth surface, and are chemically inert, resulting in a small number of bonding sites between inorganic powders and the surface of virgin aramid 1313 fibers. Step S1 uses waste aramid 1313 as raw material, including uncoated and unadhesive-coated pure waste aramid 1313 protective clothing, waste aramid 1313 nonwoven fabric, and aramid 1313 production scraps. This step pre-treats and fibrillates the waste aramid 1313, preparing a recycled aramid 1313 chopped fiber slurry. The recycled aramid 1313 chopped fiber slurry obtained in this step is used as the raw material prepared in step S4.

[0063] Specifically, after selecting the waste aramid 1313 raw material, the waste aramid 1313 is cut into blocks to obtain waste aramid 1313 block material.

[0064] After the waste aramid 1313 blocks were prepared, they were subjected to ultrasonic water cleaning to obtain ultrasonically cleaned waste aramid 1313. During the ultrasonic water cleaning process, pure water was used as the cleaning medium, the liquid-to-solid ratio of the cleaning system was controlled at 20:1, the ultrasonic power was controlled at 300W to 500W, and the cleaning time was controlled at 15min to 25min. During the ultrasonic water cleaning process, the cavitation effect generated by ultrasonic vibration acted on the surface of the waste aramid 1313 blocks, causing water-soluble impurities adhering to the surface of the waste aramid 1313 blocks to separate from the waste aramid 1313 blocks.

[0065] After ultrasonic water cleaning, the waste aramid 1313, which had undergone ultrasonic water cleaning, was subjected to alkaline solution degreasing cleaning to obtain waste aramid 1313 after alkaline solution degreasing cleaning. During the alkaline solution degreasing cleaning process, a sodium carbonate aqueous solution or a sodium bicarbonate aqueous solution with a mass fraction of 2% to 5% was used as the cleaning medium. The temperature of the cleaning system was controlled at 40℃ to 50℃, and the cleaning time was controlled at 20 min to 30 min. During the alkaline solution degreasing cleaning process, the alkaline medium reacts with the non-water-soluble oily impurities adhering to the surface of the waste aramid 1313 after ultrasonic water cleaning through saponification and emulsification, causing the non-water-soluble oily impurities to separate from the waste aramid 1313 after ultrasonic water cleaning.

[0066] After the alkaline solution degreasing and cleaning is completed, the waste aramid 1313 that has been degreased and cleaned with alkaline solution is rinsed with pure water to obtain the rinsed waste aramid 1313.

[0067] After rinsing, the rinsed waste aramid 1313 is placed in an environment of 80℃ to 90℃ for drying. The drying process continues until the moisture content of the rinsed waste aramid 1313 is ≤3%, thus obtaining pretreated waste aramid 1313.

[0068] Fibrillation refers to the morphological changes in polymer fibers caused by external forces, resulting in the formation of tiny tendrils, barbs, or micropores on the surface. After the pretreatment process of waste aramid 1313, the pretreated waste aramid 1313 undergoes multi-stage shearing to obtain sheared aramid 1313 fibers. The multi-stage shearing process is completed using a two-stage shearing machine. The pretreated waste aramid 1313 is fed into the two-stage shearing machine. The blade gap is controlled at 5mm to 8mm in the first-stage shearing process and 2mm to 3mm in the second-stage shearing process. The rotation speed of the two-stage shearing machine is controlled at 1000r / min to 1200r / min. During the multi-stage shearing process, the shearing force generated by the rotating blades acts on the pretreated waste aramid 1313, shortening its length and forcibly disrupting the orientation of the surface macromolecular chains.

[0069] After multi-stage shearing, the sheared aramid 1313 fibers are mechanically opened using a carding opener to obtain opened aramid 1313 fibers. During the mechanical opening process, the rotation speed of the carding opener is controlled between 600 r / min and 800 r / min. During the mechanical opening process, the combing force generated by the carding needles acts on the sheared aramid 1313 fibers, further disrupting the highly oriented macromolecular chain structure of the sheared aramid 1313 fibers and inducing fibrillation in the sheared aramid 1313 fibers.

[0070] After mechanical opening, the opened aramid 1313 fibers are screened using a double-layer vibrating screener to collect fibers with a length of 2.0 mm to 3.5 mm, thus obtaining fibrillated aramid 1313 chopped fibers.

[0071] After collecting the fibrillated aramid 1313 chopped fibers, the fibers are mixed with pure water to prepare a fiber suspension with a mass concentration of 1% to 2%. The fiber suspension is then placed in a high-speed dispersion device, with the dispersion speed controlled at 12000 r / min to 15000 r / min and the dispersion time controlled at 15 min to 30 min, to complete the high-speed dissociation of the fibers and obtain regenerated aramid 1313 chopped fiber slurry.

[0072] After step S1, the initial waste aramid 1313 raw material is transformed into a slurry of recycled aramid 1313 chopped fibers of uniform length and dispersed in individual strands. The fiber surface in the recycled aramid 1313 chopped fiber slurry changes from the dense morphology of the waste aramid 1313 raw material to a rough, porous morphology with distributed nanoscale barbs. The rough, porous morphology and nanoscale barb structure of the recycled aramid 1313 chopped fiber surface increase the contact area of ​​the fiber surface, providing contact sites for the polymer-coated powder introduced in subsequent processes.

[0073] In the current process of preparing aramid paper for gamma rays protection, a single-component inorganic shielding powder is used as the radiation functional phase. The shielding effect of the inorganic shielding powder on gamma rays is based on the photoelectric effect and Compton scattering of the outer electrons of the constituent element atoms with the incident gamma ray photons. Each element has a fixed K-shell characteristic absorption edge, which refers to the minimum incident photon energy required for the K-shell electrons of an element's atom to undergo the photoelectric effect. The gamma ray absorption spectrum of a single element has a weak absorption region within a specific energy range. In this step, bismuth tungstate powder is prepared through a hydrothermal reaction. The bismuth tungstate powder is then mixed and ground with gadolinium metal powder to prepare a composite shielding powder. The composite shielding powder obtained in this step is used as the raw material for modification in step S3.

[0074] Specifically, the preparation of bismuth tungstate powder is carried out first. Bismuth nitrate is dissolved in dilute nitric acid to obtain an acidic bismuth nitrate solution. After the acidic bismuth nitrate solution is prepared, an aqueous solution of sodium tungstate is added to it to obtain a mixed salt solution. After the mixed salt solution is prepared, the pH value is adjusted to 5.0 to 5.5 to obtain a pH-adjusted mixed salt solution. After the pH-adjusted mixed salt solution is prepared, hexadecyltrimethylammonium bromide and polyethylene glycol are added to it to obtain a bismuth tungstate precursor solution. Hexadecyltrimethylammonium bromide and polyethylene glycol act as morphology modifiers, adsorbing onto specific crystal faces of the bismuth tungstate grains during the hydrothermal reaction, thereby controlling the growth direction and exposed crystal faces of the bismuth tungstate grains.

[0075] After the bismuth tungstate precursor solution is prepared, it is placed in a hydrothermal reactor and sealed. Following sealing, the reactor is placed in a heating environment, with the temperature controlled between 160°C and 200°C, and the reaction time controlled between 12 and 24 hours. During the hydrothermal reaction, the high-temperature, high-pressure, and sealed environment lowers the dielectric constant of water, increasing the solubility and reactivity of the precursor solute in the aqueous phase. Bismuth ions and tungstate ions in the precursor solution undergo a dissolution-recrystallization process, generating bismuth tungstate crystals. The hydrothermal reaction temperature can be set to 180°C, and the reaction time can be set to 18 hours.

[0076] After the hydrothermal reaction time reaches the set value, heating is stopped, and the hydrothermal reactor is allowed to cool naturally to room temperature before the hydrothermal reaction product is removed. After removal, the hydrothermal reaction product is sequentially centrifuged, washed with water, and then washed with anhydrous ethanol to obtain a washed wet bismuth tungstate. The washed wet bismuth tungstate is then dried to obtain bismuth tungstate powder.

[0077] After the bismuth tungstate powder is prepared, it is weighed and mixed with gadolinium metal powder to obtain a powder mixture. The particle size of the gadolinium metal powder is 50 nm to 200 nm, and the mass ratio of bismuth tungstate powder to gadolinium metal powder is 3:1 to 6:1. The constituent elements of bismuth tungstate powder include bismuth and tungsten, such as... Figure 2 As shown, the characteristic absorption edge of the K-layer of bismuth is 90.5 keV, and that of tungsten is 69.5 keV. The gadolinium powder is composed of gadolinium, which has a characteristic absorption edge of 50.2 keV in its K-layer. After the powder mixture is prepared, it is ground to ensure uniform mixing of the bismuth tungstate powder and the gadolinium powder, resulting in a composite shielding powder.

[0078] After step S2, the initial bismuth nitrate and sodium tungstate raw materials are transformed into composite shielding powder with a particle size of 100 nm to 500 nm. The composite shielding powder contains three high atomic number elements: bismuth, tungsten, and gadolinium. The characteristic absorption edges of the K-layer of these three elements correspond to different energy ranges of gamma-ray photons. When the composite shielding powder interacts with incident gamma rays, the different constituent elements undergo photoelectric effect and Compton scattering with the gamma-ray photons in their respective energy ranges, resulting in attenuation of the incident gamma rays.

[0079] Inorganic composite shielding powders possess high surface energy. Direct addition of these powders to aqueous pulp systems easily leads to particle agglomeration. Furthermore, a stable interfacial bond cannot be formed between the inorganic composite shielding powders and organic aramid fibers. Step S3 uses the composite shielding powders prepared in step S2 as raw materials and employs an aqueous polyurethane elastomer emulsion to perform interfacial modification treatment, resulting in modified composite shielding powders. The modified composite shielding powders obtained in this step are used as raw materials for the preparation of the protective aramid pulp in step S4.

[0080] Specifically, the interface modifier used in this step is an aqueous polyurethane elastomer emulsion. The aqueous polyurethane elastomer emulsion has a heat resistance temperature ≥220℃, a solid content of 20% to 40%, and a particle size of 50nm to 200nm.

[0081] After selecting the waterborne polyurethane elastomer emulsion, the composite shielding powder prepared in step S2 is added to the waterborne polyurethane elastomer emulsion, and the mass ratio of the composite shielding powder to the solids in the waterborne polyurethane elastomer emulsion is controlled to be 2:1 to 5:1 to obtain the dispersion mixture system.

[0082] After the mixture to be dispersed is prepared, it is subjected to ultrasonic dispersion for 20 to 30 minutes. During ultrasonic dispersion, the cavitation effect generated by ultrasonic vibration acts on the mixture to be dispersed, breaking the soft agglomerate structure of the composite shielding powder and enabling the composite shielding powder to achieve initial dispersion in the waterborne polyurethane elastomer emulsion.

[0083] After ultrasonic dispersion, the system to be dispersed is mechanically stirred at a speed of 400 to 600 rpm for 2 to 3 hours. During the mechanical stirring process, continuous shear force acts on the system, causing the molecular chains in the waterborne polyurethane elastomer emulsion to fully extend. The extended waterborne polyurethane molecular chains then adsorb and coat the surface of the composite shielding powder particles.

[0084] After mechanical stirring, the mixed system is placed in an environment of 60°C to 70°C for drying to obtain dried modified powder block material.

[0085] After the drying process is completed, the dried modified powder block material is ground to obtain modified composite shielding powder.

[0086] After step S3, the initial composite shielding powder is transformed into a modified composite shielding powder with a particle size of 100 nm to 500 nm. The inorganic particle surface of the modified composite shielding powder is coated with a uniform and dense polyurethane film. The surface energy of the polyurethane-coated modified composite shielding powder is lower than that of the unmodified composite shielding powder. The dispersion stability of the modified composite shielding powder in an aqueous system is higher than that of the unmodified composite shielding powder. The surface of the modified composite shielding powder has polar groups from the waterborne polyurethane molecular chains.

[0087] The uniformity of aramid paper formed by wet papermaking is determined by the dispersion state of each component in the pulp system. Step S4 uses the recycled aramid 1313 chopped fiber pulp prepared in step S1 and the modified composite shielding powder prepared in step S3 as raw materials, combined with aramid precipitated fibers, to prepare protective aramid pulp. Aramid precipitated fibers are polymeric materials with irregular thin-film microstructures formed by aramid polymers under high shear forces in a coagulation bath, and are used as a binder component in the aramid papermaking system. The protective aramid pulp obtained in this step serves as the raw material for wet papermaking and hot pressing in step S5.

[0088] Specifically, the recycled aramid 1313 chopped fiber slurry prepared in step S1 is mixed with aramid precipitated fibers, and the mass ratio of the dry weight of the recycled aramid 1313 chopped fiber slurry to the dry weight of the aramid precipitated fibers is controlled to be 5:5 to 6:4 to obtain a fiber initial mixture.

[0089] After the fiber premix is ​​prepared, pure water is added to the fiber premix to prepare a mixed slurry with a mass concentration of 0.05% to 0.1%.

[0090] After the mixed slurry is prepared, the modified composite shielding powder obtained in step S3 is added to the mixed slurry. The ratio of the dry weight of the modified composite shielding powder to the total dry weight of the aramid fibers in the mixed slurry is controlled to be 1:3 to 1:5. The total dry weight of the aramid fibers in the mixed slurry is the sum of the dry weight of the fibers in the recycled aramid 1313 chopped fiber slurry and the dry weight of the aramid precipitated fibers, thus obtaining the slurry to be dispersed.

[0091] After the pulp to be dispersed is prepared, it is subjected to high-speed mechanical stirring. The stirring speed is controlled at 800 r / min to 1200 r / min, and the stirring time is controlled at 1 h to 2 h. During the high-speed mechanical stirring process, the continuous shear force generated by the stirring paddle acts on the pulp to be dispersed, breaking up the fiber flocs and enabling the recycled aramid 1313 chopped fibers, aramid precipitated fibers, and modified composite shielding powder to achieve uniform interweaving and dispersion in the aqueous system, thus obtaining protective aramid pulp.

[0092] After step S4, recycled aramid 1313 chopped fibers, aramid precipitated fibers, and modified composite shielding powder form a homogeneously dispersed protective aramid pulp in an aqueous system. The mass concentration of the protective aramid pulp is in the range of 0.05% to 0.1%, which can form an areal density of 20 g / m³ in subsequent wet papermaking processes. 2 Up to 100g / m 2 Thin paper web. The modified composite shielding powder in the protective aramid pulp does not exhibit localized agglomeration. The recycled aramid 1313 chopped fibers, aramid precipitated fibers, and modified composite shielding powder are uniformly distributed in a three-dimensional manner in the aqueous system.

[0093] In the initial paper web formed by wet papermaking, the fiber and powder components are bonded only by intermolecular van der Waals forces, resulting in low inter-component bonding strength. Step S5 uses the protective aramid pulp prepared in step S4 as input raw material and sequentially performs wet papermaking, vacuum dehydration, forced-air pre-drying, hot pressing and setting, and natural cooling processes to prepare γ-ray protective aramid paper.

[0094] Specifically, the protective aramid pulp prepared in step S4 is placed in a wet papermaking apparatus, and a 200-mesh nylon filter is used to wet paper the protective aramid pulp to obtain an initial wet paper web.

[0095] After wet papermaking, the initial wet paper web is subjected to vacuum dewatering. The ambient pressure for vacuum dewatering is controlled at -0.08 MPa to -0.1 MPa, and the dewatering time is controlled at 5 to 10 minutes. During the vacuum dewatering process, the negative pressure environment causes the free water in the initial wet paper web to pass through the filter screen. The recycled aramid 1313 chopped fibers, aramid precipitated fibers, and modified composite shielding powder in the initial wet paper web form a fixed interwoven structure on the filter screen surface, resulting in a dewatered wet paper web.

[0096] After vacuum dehydration, the dehydrated wet paper web is placed in a forced-air drying device for pre-drying. The internal temperature of the forced-air drying device is controlled at 75℃ to 85℃, and the pre-drying time is controlled at 0.5h to 1h. During the pre-drying process, the flowing hot air carries away the residual free moisture in the dehydrated wet paper web, and the moisture content of the dehydrated wet paper web continues to decrease. The macroscopic structure of the dehydrated wet paper web remains stable, resulting in the pre-dried wet paper web.

[0097] After pre-drying, the pre-dried wet paper web is placed between parallel hot press plates of a hot press equipment. The hot press temperature is set to 260℃ to 280℃, and the applied pressure is set to 14MPa to 20MPa. After the hot press plate temperature and applied pressure reach the set values, the holding time for shaping is controlled to be 2 minutes to 8 minutes. During the hot press shaping process of the pre-dried wet paper web, the hot press temperature is close to the glass transition temperature of aramid 1313. The waterborne polyurethane coating layer on the surface of the modified composite shielding powder undergoes thermoplastic melting at the set hot press temperature, forming a viscous waterborne polyurethane material. The applied pressure of 14MPa to 20MPa in the hot press equipment exerts a continuous squeezing effect on the pre-dried wet paper web, and the viscous waterborne polyurethane material penetrates into the microfibrillated pores and barbed gaps on the surface of the recycled aramid 1313 chopped fibers. Meanwhile, the intermolecular distance between the polar urethane groups in the molecular chain of the viscous waterborne polyurethane material and the amide groups on the surface of the recycled aramid 1313 chopped fiber decreases sharply with the extrusion effect caused by the application of pressure. Intermolecular hydrogen bond rearrangement occurs between the polar urethane groups and the amide groups, forming hydrogen bond bonds.

[0098] After the holding and shaping time reaches the set value, the heating and pressurization of the hot pressing equipment is stopped, the hot-pressed paper web is taken out, and the hot-pressed paper web is placed in a room temperature environment for natural cooling to obtain γ-ray protective aramid paper.

[0099] After step S5, the components of the protective aramid pulp, which are initially dispersed in the aqueous phase, are transformed into a finished gamma-ray protective aramid paper with a fixed three-dimensional interwoven structure. Within the gamma-ray protective aramid paper, the recycled aramid 1313 chopped fibers, aramid precipitated fibers, and modified composite shielding powder form a bonded structure through mechanical anchoring and intermolecular hydrogen bonds. The structural density of the gamma-ray protective aramid paper is higher than that of the pre-dried wet paper web. The modified composite shielding powder in the gamma-ray protective aramid paper forms a stable interfacial bond with the recycled aramid 1313 chopped fibers.

[0100] Example 1:

[0101] The method for preparing gamma-ray shielding aramid paper in this embodiment includes the following steps:

[0102] S1: Waste aramid 1313 is pretreated and fibrillated into short fibers to prepare recycled aramid 1313 short fiber slurry.

[0103] Uncoated and adhesive-free waste aramid 1313 protective clothing was cut into blocks to obtain waste aramid 1313 blocks. These blocks were then subjected to ultrasonic cleaning with pure water as the cleaning medium. The liquid-to-solid ratio of the cleaning system was controlled at 20:1, the ultrasonic power at 400W, and the cleaning time at 20 minutes, resulting in waste aramid 1313 after ultrasonic cleaning. The ultrasonically cleaned waste aramid 1313 was then degreased with an alkaline solution using a 3% sodium carbonate aqueous solution as the cleaning medium. The cleaning system temperature was controlled at 45℃, and the cleaning time at 25 minutes, resulting in waste aramid 1313 after alkaline degreasing. Finally, the degreased waste aramid 1313 was rinsed with pure water to obtain rinsed waste aramid 1313. The washed waste aramid 1313 was placed in an environment of 85℃ for drying until the moisture content of the washed waste aramid 1313 was ≤3%, thus obtaining pretreated waste aramid 1313.

[0104] Pre-treated waste aramid 1313 was fed into a two-stage shearing machine for multi-stage shearing. The blade gap was controlled at 6 mm in the first stage and 2.5 mm in the second stage, with the shearing machine speed controlled at 1100 r / min, yielding sheared aramid 1313 fibers. A carding opener was then used to mechanically open the sheared aramid 1313 fibers, with the speed controlled at 700 r / min, yielding opened aramid 1313 fibers. A double-layer vibrating screen was then used to screen the opened aramid 1313 fibers, collecting fibers with a length of 2.0 mm to 3.5 mm to obtain fibrillated aramid 1313 chopped fibers. These fibrillated aramid 1313 chopped fibers were mixed with pure water to prepare a fiber suspension with a mass concentration of 1.5%. The fiber suspension was placed in a high-speed dispersion device, and the dispersion speed was controlled at 13,500 r / min and the dispersion time was controlled at 22 min to complete the high-speed dissociation of the fiber and obtain the recycled aramid 1313 chopped fiber slurry.

[0105] S2: Bismuth tungstate powder is prepared by hydrothermal reaction. The bismuth tungstate powder is mixed and ground with gadolinium metal powder to obtain composite shielding powder.

[0106] Bismuth nitrate was dissolved in dilute nitric acid to obtain an acidic bismuth nitrate solution. Sodium tungstate aqueous solution was added to the acidic bismuth nitrate solution to obtain a mixed salt solution. The pH of the mixed salt solution was adjusted to 5.2 to obtain a pH-adjusted mixed salt solution. Hexadecyltrimethylammonium bromide and polyethylene glycol were added to the pH-adjusted mixed salt solution to obtain a bismuth tungstate precursor solution. The bismuth tungstate precursor solution was placed in a hydrothermal reactor, which was then sealed. The sealed hydrothermal reactor was placed in a heating environment, and the temperature of the hydrothermal reaction system was controlled at 180℃ for 18 hours. After the hydrothermal reactor cooled naturally to room temperature, the hydrothermal reaction product was removed. The hydrothermal reaction product was sequentially centrifuged, washed with water, and washed with anhydrous ethanol to obtain a washed wet bismuth tungstate material. The washed wet bismuth tungstate material was dried to obtain bismuth tungstate powder.

[0107] Bismuth tungstate powder and gadolinium metal powder were weighed and mixed to obtain a powder mixture. The particle size of the gadolinium metal powder was 50 nm to 200 nm, and the mass ratio of bismuth tungstate powder to gadolinium metal powder was 3.0:1. The powder mixture was then ground to obtain a composite shielding powder.

[0108] S3: The composite shielding powder is added to an aqueous polyurethane emulsion, and the interface is modified by dispersion and stirring. After drying and grinding, the modified composite shielding powder is obtained.

[0109] Aqueous polyurethane elastomer emulsion was used as an interface modifier. The emulsion had a heat resistance temperature ≥220℃, a solid content of 30%, and a particle size of 50nm to 200nm. The composite shielding powder prepared in step S2 was added to the emulsion, and the mass ratio of the composite shielding powder to the solids in the emulsion was controlled at 3.5:1 to obtain the dispersion mixture. The mixture was ultrasonically dispersed for 25 minutes. After ultrasonic dispersion, the mixture was mechanically stirred at 500 r / min for 2.5 hours. The stirred mixture was then dried at 65℃ to obtain dried modified powder blocks. The dried modified powder blocks were then ground to obtain the modified composite shielding powder.

[0110] S4: Mix recycled aramid 1313 chopped fiber pulp with precipitated fiber to prepare a mixed pulp, add modified composite shielding powder and stir to disperse to obtain protective aramid pulp.

[0111] The recycled aramid 1313 chopped fiber slurry prepared in step S1 is mixed with aramid precipitated fibers, and the mass ratio of the dry weight of the recycled aramid 1313 chopped fiber slurry to the dry weight of the aramid precipitated fibers is controlled at 6:4 to obtain a fiber initial mixture. Pure water is added to the fiber initial mixture to prepare a mixed slurry with a mass concentration of 0.08%. The modified composite shielding powder prepared in step S3 is added to the mixed slurry, and the ratio of the dry weight of the modified composite shielding powder to the total dry weight of the aramid fibers in the mixed slurry is controlled at 1:4 to obtain the slurry to be dispersed. The slurry to be dispersed is subjected to high-speed mechanical stirring, with the stirring speed controlled at 1000 r / min and the stirring time controlled at 1.5 h to obtain protective aramid pulp.

[0112] S5: The protective aramid pulp is wet-processed, dehydrated, and pre-dried to form a wet paper web, which is then hot-pressed and cooled to obtain γ-ray protective aramid paper.

[0113] The protective aramid pulp prepared in step S4 is placed in a wet papermaking apparatus and wet-processed using a 200-mesh nylon filter to obtain an initial wet paper web. The initial wet paper web is then subjected to vacuum dewatering, with the ambient pressure controlled at -0.09 MPa and the dewatering time controlled at 8 minutes, resulting in a dewatered wet paper web. The dewatered wet paper web is then placed in a forced-air drying apparatus for pre-drying, with the internal ambient temperature controlled at 80°C and the pre-drying time controlled at 45 minutes, resulting in a pre-dried wet paper web. The pre-dried wet paper web is then placed between parallel hot press plates in a hot press apparatus. The hot press temperature is set to 270°C and the applied pressure is set to 16 MPa. After the hot press plate temperature and applied pressure reach the set values, the pressure holding and shaping time is controlled at 5 minutes. After the holding and shaping time reaches the set value, the heating and pressurization of the hot pressing equipment is stopped, the hot-pressed paper web is taken out, and the hot-pressed paper web is placed in a room temperature environment for natural cooling to obtain γ-ray protective aramid paper.

[0114] Example 2:

[0115] The difference between this embodiment and Embodiment 1 is that in step S2, the mass ratio of bismuth tungstate powder to gadolinium metal powder is 4.5:1. All other steps, process parameters, and raw material ratios are the same as in Embodiment 1.

[0116] Example 3:

[0117] The difference between this embodiment and Embodiment 1 is that in step S2, the mass ratio of bismuth tungstate powder to gadolinium metal powder is 6.0:1. All other steps, process parameters, and raw material ratios are the same as in Embodiment 1.

[0118] Example 4:

[0119] The difference between this embodiment and Embodiment 1 is that:

[0120] In step S2, the mass ratio of bismuth tungstate powder to gadolinium metal powder is 4.5:1;

[0121] In step S3, the mass ratio of the composite shielding powder to the solids in the waterborne polyurethane elastomer emulsion is 2.0:1.

[0122] The remaining steps, process parameters, and raw material ratios are the same as in Example 1.

[0123] Example 5:

[0124] The difference between this embodiment and Embodiment 1 is that:

[0125] In step S2, the mass ratio of bismuth tungstate powder to gadolinium metal powder is 4.5:1;

[0126] In step S3, the mass ratio of the composite shielding powder to the solids in the waterborne polyurethane elastomer emulsion is 5.0:1.

[0127] The remaining steps, process parameters, and raw material ratios are the same as in Example 1.

[0128] Comparative Example 1:

[0129] The difference between this comparative example and Example 1 is that in step S2, the mass ratio of bismuth tungstate powder to gadolinium metal powder is 2.9:1. All other steps, process parameters, and raw material ratios are the same as in Example 1.

[0130] Comparative Example 2:

[0131] The difference between this comparative example and Example 1 is that in step S2, the mass ratio of bismuth tungstate powder to gadolinium metal powder is 6.1:1. All other steps, process parameters, and raw material ratios are the same as in Example 1.

[0132] Comparative Example 3:

[0133] The difference between this comparative example and Example 1 is that:

[0134] In step S2, the mass ratio of bismuth tungstate powder to gadolinium metal powder is 4.5:1;

[0135] In step S3, the mass ratio of the composite shielding powder to the solids in the waterborne polyurethane elastomer emulsion is 1.9:1.

[0136] The remaining steps, process parameters, and raw material ratios are the same as in Example 1.

[0137] Comparative Example 4:

[0138] The difference between this comparative example and Example 1 is that:

[0139] In step S2, the mass ratio of bismuth tungstate powder to gadolinium metal powder is 4.5:1;

[0140] In step S3, the mass ratio of the composite shielding powder to the solids in the waterborne polyurethane elastomer emulsion is 5.1:1.

[0141] The remaining steps, process parameters, and raw material ratios are the same as in Example 1.

[0142] Comparative Example 5:

[0143] The difference between this comparative example and Example 1 is that:

[0144] Step S1 is replaced by the preparation of virgin aramid 1313 chopped fiber slurry. The specific process is as follows: using virgin aramid 1313 fiber as raw material, the virgin aramid 1313 fiber is cut into short fibers of 2.0 mm to 3.5 mm, and water is added to prepare a suspension with a mass concentration of 1.5%. The suspension is placed in a high-speed dispersion device, the dispersion speed is controlled at 13500 r / min, and the dispersion time is controlled at 22 min to complete the high-speed dissociation of the fiber and obtain virgin aramid 1313 chopped fiber slurry.

[0145] In step S2, the mass ratio of bismuth tungstate powder to gadolinium metal powder is 4.5:1.

[0146] The remaining steps, process parameters, and raw material ratios are the same as in Example 1.

[0147] Comparative Example 6:

[0148] The difference between this comparative example and Example 1 is that:

[0149] In step S2, bismuth tungstate powder is prepared by high-temperature calcination instead of bismuth tungstate powder prepared by hydrothermal method. The process of preparing bismuth tungstate powder by high-temperature calcination is as follows: bismuth nitrate and sodium tungstate are mixed and ground in stoichiometric ratio, placed in a muffle furnace, calcined at 600℃ for 4 hours, and ground after natural cooling to obtain bismuth tungstate powder.

[0150] In step S2, the mass ratio of bismuth tungstate powder to gadolinium metal powder is 4.5:1.

[0151] The remaining steps, process parameters, and raw material ratios are the same as in Example 1.

[0152] Experimental Example 1:

[0153] The test samples in this experiment were the γ-ray shielding aramid paper prepared in Examples 1 to 5 and Comparative Examples 1 to 6. The target areal density of all γ-ray shielding aramid paper samples was controlled to be 50 g / m³. 2 The following tests were performed on the test samples:

[0154] 1. Tensile Strength Test: Refer to GB / T 12914-2018 "Determination of Tensile Strength of Paper and Paperboard (Constant Rate Tensile Test)". A universal testing machine was used as the testing equipment, and the testing environment was a standard laboratory environment with a temperature of 23℃ and a relative humidity of 50%. The γ-ray protective aramid paper to be tested was cut into tensile specimens with a width of 15mm and a length of 180mm. The tensile rate of the universal testing machine was set to 20mm / min, and the clamp spacing was set to 100mm. Five parallel tensile specimens were prepared for each group of γ-ray protective aramid paper samples. After the test, the arithmetic mean of the test results of all parallel specimens was taken as the final test result of the tensile strength of that group of samples.

[0155] 2. Gamma-ray Shielding Performance Test: The gamma-ray shielding performance test was conducted using a standard gamma-ray radiation source covering the characteristic energy range of 50 keV to 100 keV, in conjunction with a high-purity germanium gamma-ray spectrometer. During the test, the gamma-ray shielding aramid paper to be tested was fixed in the collimated optical path between the standard gamma-ray radiation source and the detector of the high-purity germanium gamma-ray spectrometer. The incident gamma-ray count rate without a sample and the transmitted gamma-ray count rate with a sample were measured respectively. The gamma-ray shielding efficiency of the sample was calculated based on the incident and transmitted gamma-ray count rates. Each group of gamma-ray shielding aramid paper samples was tested three times, and the arithmetic mean of all test results was taken as the final test result of the characteristic gamma-ray shielding efficiency of that group of samples.

[0156] 3. Test for folding endurance and powder shedding performance: Refer to GB / T 457-2008 "Determination of folding endurance of paper and paperboard (MIT method)". The folding endurance and powder shedding performance test uses an MIT folding endurance meter as the folding treatment equipment and an electronic analytical balance with an accuracy of 0.1 mg as the weighing device. The test environment is a standard laboratory environment with a temperature of 23℃ and a relative humidity of 50%. The γ-ray protective aramid paper to be tested is cut into folding endurance samples with a width of 15 mm and a length of 140 mm. The spring tension of the MIT folding endurance meter is set to 9.8 N, and the folding angle is set to ±135°. The folding endurance samples are subjected to 100 consecutive folds. The tester uses an electronic analytical balance to weigh the dry weight of the folding endurance samples before and after the folding endurance treatment, and calculates the mass loss rate of the samples based on the mass difference before and after the folding endurance treatment. Five parallel folding endurance test specimens were prepared for each group of γ-ray protected aramid paper samples. The arithmetic mean of the test results of all parallel specimens was taken as the final test result of the mass loss of the sample after 100 folds at MIT.

[0157] The results are shown in the table below:

[0158] Table 1

[0159] Group Characteristic gamma-ray shielding efficiency (%) Tensile strength (MPa) MIT mass loss (%) after 100 folds Example 1 86.2 58.5 0.8 Example 2 92.5 61.2 0.6 Example 3 85.8 60.1 0.7 Example 4 91.0 52.3 0.2 Example 5 91.8 55.6 1.9 Comparative Example 1 82.1 45.2 1.1 Comparative Example 2 78.5 60.5 0.6 Comparative Example 3 90.5 49.6 0.2 Comparative Example 4 87.6 50.1 8.5 Comparative Example 5 90.2 62.0 5.2 Comparative Example 6 81.3 53.4 1.5

[0160] When gamma rays interact with matter, the probability of the photoelectric effect occurring is directly proportional to the fifth power of the atomic number of the interacting element and inversely proportional to the cube of the energy of the incident gamma-ray photon. Each element has a fixed K-shell characteristic absorption edge, which is the minimum incident photon energy required for the K-shell electrons of an element's atom to undergo the photoelectric effect. When the energy of the incident gamma-ray photon approaches the K-shell characteristic absorption edge of an element, the photoelectric absorption cross-section of that element for incident gamma-ray photons increases. The K-shell characteristic absorption edge of bismuth is 90.5 keV, that of tungsten is 69.5 keV, and that of gadolinium is 50.2 keV. These three K-shell characteristic absorption edges correspond to different gamma-ray photon energy ranges. The change in the mass ratio of bismuth tungstate powder to gadolinium metal powder will alter the mass proportions of bismuth, tungsten, and gadolinium in the powder system composed of bismuth tungstate powder and gadolinium metal powder, thereby changing the absorption coverage range of the powder system for γ-rays in the 50keV to 100keV energy range.

[0161] In Examples 1, 2, and 3, the mass ratios of bismuth tungstate powder to gadolinium powder were 3.0:1, 4.5:1, and 6.0:1, respectively. The characteristic gamma-ray shielding efficiencies for Examples 1, 2, and 3 were 86.2%, 92.5%, and 85.8%, respectively. The characteristic gamma-ray shielding efficiencies showed a trend of first increasing and then decreasing with the increase of the mass ratio of bismuth tungstate powder to gadolinium powder. When the mass ratio of bismuth tungstate powder to gadolinium powder was 3.0:1, the mass proportion of gadolinium in the powder system was relatively high, and the high absorption range corresponding to gadolinium was concentrated around 50.2 keV. The absorption ratio of gamma rays in the 60 keV to 90 keV range was insufficient, resulting in a relatively low characteristic gamma-ray shielding efficiency for the final gamma-ray-protective aramid paper. When the mass ratio of bismuth tungstate powder to gadolinium powder is 4.5:1, the mass proportions of bismuth, tungsten, and gadolinium in the powder system are well-matched. The high absorption range corresponding to the characteristic absorption edges of the K-layer of these three elements forms a continuous coverage, resulting in graded absorption of gamma rays in the 50keV to 100keV energy range. Consequently, the characteristic gamma-ray shielding rate of the resulting gamma-ray-protective aramid paper reaches its highest value. Conversely, when the mass ratio of bismuth tungstate powder to gadolinium powder is 6.0:1, the mass proportion of gadolinium in the powder system is relatively low. This reduces the absorption capacity of the powder system for gamma rays in the 50keV to 60keV range, creating a weak absorption region. Consequently, the characteristic gamma-ray shielding rate of the resulting gamma-ray-protective aramid paper decreases.

[0162] In Comparative Example 1, the mass ratio of bismuth tungstate powder to gadolinium powder was 2.9:1. The tensile strength of Comparative Example 1 was 45.2 MPa, which was lower than the 61.2 MPa of Example 2. This change is due to the fact that excessive nano-sized gadolinium powder has high surface energy and is prone to particle agglomeration in the pulp system. Agglomerated particles form stress concentration points in the paper matrix structure of the γ-ray shielding aramid paper. When subjected to tension, the γ-ray shielding aramid paper is prone to structural fracture at these stress concentration points, ultimately leading to a decrease in tensile strength. Simultaneously, the local agglomeration of nanoparticles results in uneven distribution of the shielding elements within the paper matrix, forming microscopic radiation penetration channels, which in turn leads to a simultaneous decrease in the characteristic γ-ray shielding rate. In Comparative Example 2, the mass ratio of bismuth tungstate powder to gadolinium powder was 6.1:1. The characteristic γ-ray shielding rate of Comparative Example 2 was 78.5%, which was lower than the 92.5% of Example 2. The reason for this change is that the mass proportion of gadolinium in the powder system is further reduced, the range of weak absorption in the 50keV to 60keV range is expanded, the attenuation ability of the powder system for γ-rays in this energy range is reduced, and ultimately the characteristic γ-ray shielding rate is reduced.

[0163] The coating effect of waterborne polyurethane elastomer emulsion on composite shielding powder can reduce the surface energy of the composite shielding powder and improve its dispersion uniformity in the pulp system. During hot pressing and setting, the heated and molten waterborne polyurethane material can penetrate into the microfibrillated pores on the surface of recycled aramid 1313 chopped fibers, forming a bonding bridge between the composite shielding powder and the recycled aramid 1313 chopped fibers. The polar urethane groups on the waterborne polyurethane molecular chain can form intermolecular hydrogen bonds with the amide groups on the surface of the aramid fibers, improving the interfacial bonding strength between the composite shielding powder and the aramid fibers. Changes in the mass ratio of solids in the composite shielding powder and the waterborne polyurethane elastomer emulsion will alter the integrity of the coating layer on the surface of the composite shielding powder, change the mass proportion of the polyurethane binder phase in the γ-ray protective aramid paper base system, and thus affect the shielding performance, tensile strength, and folding endurance of the γ-ray protective aramid paper.

[0164] In Examples 4, 2, and 5, the mass ratios of the composite shielding powder to the solids in the waterborne polyurethane elastomer emulsion were 2.0:1, 3.5:1, and 5.0:1, respectively. The characteristic gamma-ray shielding efficiencies for Examples 4, 2, and 5 were 91.0%, 92.5%, and 91.8%, respectively, showing relatively small fluctuations. The tensile strengths for Examples 4, 2, and 5 were 52.3 MPa, 61.2 MPa, and 55.6 MPa, respectively, showing a trend of first increasing and then decreasing with increasing mass ratios of the composite shielding powder to the solids in the waterborne polyurethane elastomer emulsion. The mass loss of the MIT (ultrasound-resistant material) after 100 folds for Examples 4, 2, and 5 were 0.2%, 0.6%, and 1.9%, respectively, showing a continuous increasing trend with increasing mass ratios of the composite shielding powder to the solids in the waterborne polyurethane elastomer emulsion.

[0165] When the mass ratio of the composite shielding powder to the solids in the waterborne polyurethane elastomer emulsion is 2.0:1, the mass proportion of waterborne polyurethane solids in the system is relatively high. The polyurethane coating layer can completely cover the particle surface of the composite shielding powder, and the interfacial bonding strength between the composite shielding powder and the aramid fiber is high. Therefore, the final γ-ray shielding aramid paper has a low MIT (Mean Intensity Tolerance) mass loss after 100 folds. However, excessive waterborne polyurethane solids will reduce the mass proportion of the aramid fiber skeleton in the paper-based system, decreasing the proportion of the main load-bearing structure of the paper base. Therefore, the tensile strength of the final γ-ray shielding aramid paper is lower than that corresponding to Example 2. Since the waterborne polyurethane material is a non-shielding component, excessive waterborne polyurethane solids will reduce the mass proportion of shielding elements per unit area. Therefore, the characteristic γ-ray shielding rate of the final γ-ray shielding aramid paper is slightly lower than that corresponding to Example 2.

[0166] When the mass ratio of the composite shielding powder to the solids in the waterborne polyurethane elastomer emulsion is 3.5:1, the amount of waterborne polyurethane solids can effectively coat the surface of the composite shielding powder particles without excessively squeezing the mass ratio of the aramid fiber skeleton in the paper-based system. A balance is formed between interfacial bonding strength, material mechanical strength, and shielding performance, and the comprehensive performance parameters of the resulting γ-ray protective aramid paper are in the optimal range.

[0167] When the mass ratio of the composite shielding powder to the solids in the aqueous polyurethane elastomer emulsion is 5.0:1, the mass proportion of aqueous polyurethane solids in the system is relatively low. This results in some composite shielding powder particles failing to form a complete coating layer, and a decrease in the interfacial bonding strength between the composite shielding powder and the aramid fiber. Consequently, the final γ-ray protective aramid paper exhibits an increased mass loss after 100 folds. Simultaneously, the incompletely coated composite shielding powder is prone to particle agglomeration. Agglomerated particles disrupt the continuity of the paper matrix structure, thus the tensile strength of the final γ-ray protective aramid paper is lower than that corresponding to Example 2.

[0168] In Comparative Example 3, the mass ratio of the composite shielding powder to the solids in the waterborne polyurethane elastomer emulsion was 1.9:1. The tensile strength of Comparative Example 3 was 49.6 MPa, a decrease from 61.2 MPa in Example 2. This change is due to the further increase in the mass proportion of waterborne polyurethane solids in the system, the further decrease in the mass proportion of the aramid fiber skeleton in the paper-based system, and the continuous decrease in the structural load-bearing capacity of the paper base, ultimately leading to a decrease in tensile strength. In Comparative Example 4, the mass ratio of the composite shielding powder to the solids in the waterborne polyurethane elastomer emulsion was 5.1:1. The mass loss of MIT after 100 folds in Comparative Example 4 was 8.5%, a significant increase from 0.6% in Example 2. The reason for this change is that the amount of waterborne polyurethane solids in the system is insufficient, making it impossible to form a continuous coating layer on the surface of the composite shielding powder particles, nor to form an effective bonding bridge between the composite shielding powder and the aramid fiber. After repeated folding, the interface between the composite shielding powder and the aramid fiber peels off, resulting in a large amount of powder falling off. The characteristic gamma-ray shielding rate of Comparative Example 4 is 87.6%, which is lower than the 92.5% of Example 2. This is because the powder falling off leads to a decrease in the shielding element content per unit area of ​​the gamma-ray protective aramid paper, thus reducing its attenuation ability for gamma rays.

[0169] Comparative Example 5 used virgin aramid 1313 chopped fiber slurry instead of the recycled aramid 1313 chopped fiber slurry prepared in step S1 of Example 1. The remaining process parameters of Comparative Example 5 were completely consistent with those of Example 2. The mass loss after 100 folds (MIT) in Comparative Example 5 was 5.2%, a significant increase compared to 0.6% in Example 2. The tensile strength of Comparative Example 5 was 62.0 MPa, similar to 61.2 MPa in Example 2. This change is due to the high crystallinity of virgin aramid 1313 fibers, resulting in a dense and smooth surface lacking the pores and barbs formed by microfibrillation. During hot pressing, the molten waterborne polyurethane material could not form a stable bond with the virgin aramid 1313 fibers through mechanical anchoring, relying only on limited hydrogen bonding. After repeated folding, the interface between the composite shielding powder and the virgin aramid 1313 fibers easily peeled off, leading to significant powder shedding. Example 2 uses recycled aramid 1313 chopped fibers with microfibrillated pores and barbed structures on the surface, which can provide mechanical anchoring sites for molten waterborne polyurethane materials. The mechanical anchoring effect and the hydrogen bonding between waterborne polyurethane and aramid fibers work synergistically to improve the interfacial bonding stability between the composite shielding powder and aramid fibers and reduce the amount of powder falling off after repeated folding.

[0170] Comparative Example 6 used bismuth tungstate powder prepared by high-temperature calcination, replacing the bismuth tungstate powder prepared by hydrothermal method in step S2 of Example 1. The remaining process parameters of Comparative Example 6 were completely consistent with those of Example 2. The characteristic gamma-ray shielding rate of Comparative Example 6 was 81.3%, a decrease from 92.5% in Example 2. The tensile strength of Comparative Example 6 was 53.4 MPa, a decrease from 61.2 MPa in Example 2. This change is due to the fact that the bismuth tungstate powder prepared by high-temperature calcination has a wide particle size distribution and easily forms hard agglomerates. When mixed with gadolinium powder, it cannot form a uniformly dispersed powder system, resulting in insufficient shielding element content in some areas of the gamma-ray shielding aramid paper, leading to areas where gamma rays penetrate and a decrease in overall shielding performance. Simultaneously, the powder agglomerates form stress concentration points in the paper structure. When the gamma-ray shielding aramid paper is subjected to tension, structural fracture easily occurs at these stress concentration points, resulting in a decrease in tensile strength. Bismuth tungstate powder prepared by hydrothermal method has uniform particle size distribution and good particle dispersibility. It can form a uniform composite powder system with gadolinium powder, ensuring the uniformity of the shielding performance of aramid paper for gamma ray protection, while reducing stress concentration caused by powder agglomeration and maintaining the mechanical properties of the paper base structure.

[0171] Based on the preferred embodiments of the present invention described above, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A method for preparing gamma-ray shielding aramid paper using waste aramid 1313, characterized in that, Includes the following steps: S1: Pre-treat and fibrillate waste aramid 1313 to prepare recycled aramid 1313 chopped fiber slurry. S2: Bismuth tungstate powder is prepared by hydrothermal reaction, and the bismuth tungstate powder is mixed and ground with gadolinium metal powder to obtain composite shielding powder; S3: The composite shielding powder is added to an aqueous polyurethane emulsion, and the interface is modified by dispersion and stirring. After drying and grinding, the modified composite shielding powder is obtained. S4: The recycled aramid 1313 chopped fiber pulp is mixed with aramid precipitated fiber to prepare a mixed pulp, the modified composite shielding powder is added and stirred and dispersed to obtain protective aramid pulp; S5: The protective aramid pulp is wet-processed, dehydrated, and pre-dried to form a wet paper web, which is then hot-pressed and cooled to obtain γ-ray protective aramid paper.

2. The method according to claim 1, characterized in that, Step S1 specifically includes: The waste aramid 1313 was cut into blocks and then subjected to ultrasonic cleaning with water, degreasing cleaning with alkaline solution, and rinsing with pure water in sequence. After drying, the pretreated waste aramid 1313 was obtained. The pretreated waste aramid 1313 is fed into a two-stage shearing machine for multi-stage shearing, and then mechanically opened using a comb-type opening machine to induce fibrillation of the fibers. A double-layer vibrating screener was used for screening to collect fibrillated aramid 1313 short fibers with a length of 2.0 mm to 3.5 mm. The fibrillated aramid 1313 chopped fibers were mixed with pure water to form a fiber suspension, which was then subjected to high-speed dissociation to obtain the recycled aramid 1313 chopped fiber slurry.

3. The method according to claim 2, characterized in that: The alkaline solution degreasing and cleaning uses a sodium carbonate aqueous solution or sodium bicarbonate aqueous solution with a mass fraction of 2% to 5%, at a temperature of 40°C to 50°C, for a duration of 20 to 30 minutes. The drying process continues until the moisture content of the waste aramid 1313 is ≤3%; In the multi-stage shearing process, the tool gap is controlled to be 5mm to 8mm in the first-stage shearing process and 2mm to 3mm in the second-stage shearing process. The fiber suspension has a mass concentration of 1% to 2%, and the high-speed dissociation dispersion speed is 12000 r / min to 15000 r / min, with a duration of 15 min to 30 min.

4. The method according to claim 1, characterized in that, In step S2, the process of preparing bismuth tungstate powder via hydrothermal reaction includes: Bismuth nitrate is dissolved in dilute nitric acid to obtain an acidic bismuth nitrate solution, and then an aqueous sodium tungstate solution is added to obtain a mixed salt solution. Adjust the pH of the mixed salt solution to 5.0 to 5.5, and add hexadecyltrimethylammonium bromide and polyethylene glycol to obtain a bismuth tungstate precursor solution; The bismuth tungstate precursor solution was placed in a closed hydrothermal reactor and subjected to a hydrothermal reaction at 160°C to 200°C for 12 to 24 hours. After cooling, the product is removed and subjected to centrifugation, water washing, anhydrous ethanol washing, and drying to obtain the bismuth tungstate powder.

5. The method according to claim 1 or 4, characterized in that: In step S2, the particle size of the gadolinium metal powder is 50 nm to 200 nm, and the mass ratio of the bismuth tungstate powder to the gadolinium metal powder is 3:1 to 6:

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6. The method according to claim 1, characterized in that: In step S3, the waterborne polyurethane emulsion is a waterborne polyurethane elastomer emulsion with a heat resistance temperature ≥220℃, a solid content of 20% to 40%, and an emulsion particle size of 50nm to 200nm; the mass ratio of the composite shielding powder to the solids in the waterborne polyurethane emulsion is 2:1 to 5:

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7. The method according to claim 6, characterized in that, The interface modification process in step S3 specifically includes: The composite shielding powder is added to the aqueous polyurethane emulsion and subjected to ultrasonic dispersion treatment for 20 to 30 minutes. Then, mechanical stirring was carried out for 2 to 3 hours at a speed of 400 to 600 r / min. The mixed system was dried in an environment of 60°C to 70°C and then ground to obtain the modified composite shielding powder.

8. The method according to claim 1, characterized in that, In step S4: The mass ratio of the dry weight of the recycled aramid 1313 chopped fiber slurry to the dry weight of the aramid precipitated fiber is 5:5 to 6:

4. The mass concentration of the mixed slurry is 0.05% to 0.1%; The ratio of the dry weight of the modified composite shielding powder to the total dry weight of the aramid fibers in the mixed slurry is 1:3 to 1:

5. During the process of adding the modified composite shielding powder and stirring and dispersing it, the mechanical stirring speed is controlled at 800 r / min to 1200 r / min, and the duration is 1 h to 2 h.

9. The method according to claim 1, characterized in that, In step S5: The wet papermaking and dehydration process includes: wet papermaking using a 200-mesh filter, followed by vacuum dehydration for 5 to 10 minutes under an ambient pressure of -0.08 MPa to -0.1 MPa. The pre-drying process controls the internal ambient temperature to be between 75°C and 85°C, and the pre-drying time is between 0.5h and 1h.

10. The method according to claim 1, characterized in that, In step S5, the parameters for hot pressing include: hot pressing temperature of 260°C to 280°C, applied pressure of 14MPa to 20MPa, and holding time of 2min to 8min.

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