Method for improving stability and toughness of natural latex

Abstract

The invention relates to the technical field of natural latex preservation and processing, discloses a method for improving stability and toughness of natural latex, and adopts the technical scheme that bacterial cellulose is prepared into cellulose microgel and then added into the natural latex. The rheological property and the stability of natural latex are effectively improved, and the problem that high-ammonia latex is difficult to form and solidify is avoided; in addition, the cellulose microgel / natural latex composite latex film prepared by adopting an impregnation method has the advantages that the mechanical property is obviously improved, and particularly, the toughness is greatly enhanced; the invention provides an innovative and effective solution for stable preservation of natural latex under low-ammonia or even ammonia-free conditions, impregnation process optimization and latex film mechanical property improvement, and has wide application prospects and important economic values in the field of rubber product production and processing.

Description

Technical Field

[0001] The present invention belongs to the technical field of polymer material modification, and specifically relates to a method for improving the stability and toughness of natural latex. Bacterial cellulose is prepared into cellulose microgel (CM) and added to natural rubber latex to solve the technical problems of difficult stabilization of low-ammonia natural latex and difficult coagulation of high-ammonia latex. Background Art

[0002] Bacterial cellulose (BC) is a natural nanomaterial synthesized by microorganisms (such as Acetobacter xylinum). Its structure is the same as that of plant cellulose, but its performance is more excellent. Different from traditional cellulose, BC is directly produced by biological fermentation, with ultra-high purity, no lignin and hemicellulose residues, and a fiber diameter of only 20-100 nanometers, forming a unique three-dimensional network structure, which endows it with super mechanical strength (the wet tensile strength can reach more than 200 MPa), high water retention (the water retention rate exceeds 99%), and excellent biocompatibility, making bacterial cellulose fibers widely applicable in special fields.

[0003] Natural rubber latex (NRL) is widely used in the production of rubber products, but problems are prominent during storage and processing. During the storage of natural rubber latex, due to processes such as hydrolysis of lipids and phospholipids on the surface of rubber particles and acid production by microbial metabolism, the acidity of the system increases, disrupting the charge balance on the surface of NR particles, resulting in a decrease in colloidal stability, and then self-coagulation occurs, making NRL unable to be sold immediately. Industrially, ammonia is generally used as a stabilizer. Although it can improve the colloidal stability by reacting with fatty acids and inhibiting microbial growth, it will cause problems such as difficult processing of NRL, environmental pollution, and product safety, and a high ammonia content is likely to cause excessive stability of NRL, resulting in hindered coagulation during the molding of NRL products. At the same time, the method of compound preservation of ammonia and fungicides has disadvantages such as high cost, making the low-ammonia or ammonia-free stable preservation of NRL a long-standing unsolved problem in the industry. In terms of processing and product performance, the mechanical properties of the film prepared by the traditional dipping method are not good, and there are problems such as uneven film thickness and insufficient vulcanization. Existing toughening strategies for improving polymer properties, such as copolymerization, blending, and crystallization regulation, have defects such as affecting polymer properties, complex processes, high costs, and low energy efficiency when applied to natural rubber latex. Therefore, there is an urgent need to develop a technology that can improve the rheological properties of natural rubber latex, achieve stability under low-ammonia conditions, and enhance the mechanical properties of products to promote the development of the rubber product industry. Summary of the Invention

[0004] The present invention provides a method for improving the stability and toughness of natural latex. By means of a specific process, bacterial cellulose is made into cellulose microgels, and with its unique nanofiber network structure, a stable system is synergistically constructed with natural latex. This system can effectively optimize the rheological properties of natural latex, improve its storage stability, and at the same time significantly enhance the mechanical properties of the cellulose composite natural latex film prepared by the dipping process.

[0005] The present invention selects non-toxic and biodegradable bacterial cellulose as the raw material. The prepared cellulose microgels can be uniformly dispersed in natural latex, achieving stable preservation of natural latex under low-ammonia conditions while greatly improving the performance of related products, providing an innovative solution for the low-ammonia preservation of natural latex and the optimization of product performance.

[0006] The present invention provides a method for improving the stability and toughness of natural latex. Bacterial cellulose is prepared into cellulose microgels, and the cellulose microgels are added to natural rubber latex to achieve stable preservation of low-ammonia natural rubber latex and improve mechanical properties.

[0007] The preparation method of the cellulose microgels is specifically as follows: (1) Soak and wash the bacterial cellulose block in deionized water to remove all internal and external impurities and electrolytes until the TDS of the water after washing is 0 - 1 ppm. (2) Add the washed bacterial cellulose block to deionized water more than 100 times its mass. After mixing, place it in a colloid mill, adjust the gap to 1 mm, and grind it at the highest speed for 20 - 30 min. Finally, use a rotary evaporator to concentrate at 70 °C to obtain a cellulose microgel solution with a solid content of not less than 0.5%.

[0008] When adding the cellulose microgels to natural rubber latex, based on the dry weight of natural rubber latex, the mass fraction of the added cellulose microgels (dry weight) is 0.025 - 0.2%.

[0009] The present invention introduces cellulose microgel (CM) into the natural rubber latex system, successfully achieving the stable preservation of low-ammonia natural rubber latex, and at the same time significantly improving the mechanical properties of natural latex films. Analyzed from the microscopic level, the three-dimensional network structure of CM promotes various interactions such as hydrogen bonds and van der Waals forces between it and rubber particles. These interactions can effectively inhibit the agglomeration and sedimentation of latex particles, enhance the stability of the latex system, avoid the colloid instability caused by the aggregation of latex particles, and ensure the long-term stable storage of latex under low-ammonia conditions; when preparing the CM / NR composite film, the incorporation of CM changes the microscopic structure and intermolecular interactions of the NR film. The micron-scale CM has the effects of crack deflection or crack pining. When cracks are generated during the stretching process, the cracks will spread through a more tortuous path, which further increases the energy required for crack propagation. When the composite film is stressed, such as during stretching, the CM network structure dissipates energy through its own deformation, avoids stress concentration, realizes the toughening of the natural latex film, and improves its mechanical properties.

[0010] The present invention uses mechanical property testing methods such as differential scanning calorimetry (DSC) and tensile tests to evaluate the material property changes from multiple dimensions; with the help of microscopic structure characterization techniques such as field emission scanning electron microscopy (FE-SEM) and X-ray microscopy, directly observe the distribution of CM in the natural rubber latex system and composite film, and the interaction with rubber particles, providing strong evidence support for the latex stability and film toughening effect.

[0011] The present invention first prepares cellulose microgel (CM) from bacterial cellulose and adds it to natural rubber latex (NRL). As the addition amount of CM increases, the CM / NRL system gradually exhibits a yield behavior, and the yield value continuously increases. This is because of the unique size and network structure of CM, which can form a permeating network in NRL. This permeating network can effectively restrict the movement of NR particles, thereby significantly improving the stability of low-ammonia latex. Based on this, the present invention realizes the optimization of the storage and dipping processes of low-ammonia NRL.

[0012] The CM / NR nanocomposite film prepared by the dipping method in the present invention has better mechanical properties and significantly improved toughness compared with the pure NR film. At the optimum, the strength is increased by 7 MPa, the ductility is increased by 200%, the toughness is increased by 20 kJ / m³. The micron-scale CM has effects such as crack deflection, increasing the energy for crack propagation during stretching. Its "soft" characteristics participate in stretching, improving the film strength and elongation at break.

[0013] The present invention improves the rheological properties of low-ammonia natural latex, realizes low-ammonia stability, and prepares a cellulose composite natural latex film with excellent mechanical properties by adding cellulose microgel.

[0014] The present invention uses cellulose microgels to modify natural rubber latex, which has significant advantages in optimizing the dipping process, improving mechanical properties, and achieving low-ammonia preservation, and has good application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 is the particle size distribution diagram (A), optical photograph (B), and structural micrograph (C) of the prepared cellulose microgels; Figure 2 is the photograph of the natural rubber latex and cellulose composite components prepared in Examples 1-4 and Comparative Example 1; Figure 3 is the flow curve of the natural rubber latex and cellulose composite system prepared in Examples 1-4 and Comparative Example 1; Figure 4 is the yield force test result of the natural rubber latex and cellulose composite system prepared in Examples 1-4 and Comparative Example 1; Figure 5 is the thixotropic loop test result of the natural rubber latex and cellulose composite system prepared in Examples 1-4 and Comparative Example 1; Figure 6 is the small amplitude oscillatory shear test result of the natural rubber latex and cellulose composite prepared in Examples 1-4 and Comparative Example 1; Figure 7 is the tensile stress-strain curve (A), toughness comparison (B), comparison of elongation at break and tensile strength (C), five 30% strain cycle tensile tests (D), and comparison of energy dissipation based on the first and last times (E) of the impregnated film of the natural rubber latex and CM composite system prepared in Examples 5-8 and Comparative Example 2; Figure 8 is the thermal stability analysis result of the impregnated film of the natural rubber latex and cellulose composite system prepared in Examples 5-8 and Comparative Example 2; Figure 9 is the cross-sectional SEM and elemental energy spectrum of the impregnated film of the composite system prepared in Example 7. DETAILED DESCRIPTION OF THE INVENTION

[0016] The present invention will be further described in detail below with reference to the drawings. It should be noted that the protection scope of the present invention is not limited by these embodiments. The bacterial cellulose used in the embodiments of the present invention is commercially available molasses coconut fruit (Shanghai Chaohuan Food Co., Ltd.), and it is washed with deionized water until the conductivity of the washing liquid is the same as that of deionized water and TDS = 0 before use.

[0017] Abbreviations used in this article: TEMPO: 2,2,6,6 - tetramethylpiperidine oxide; TDS: total dissolved solids; CM: bacterial cellulose microgel; NRL: natural rubber latex; Based on the dry weight of the latex, CM / NR - 1, CM / NRL - 2, CM / NRL - 3, and CM / NRL - 4 are natural rubber latex systems added with 0.025%, 0.05%, 0.1%, and 0.2% cellulose microgels respectively.

[0018] The preparation method of the cellulose microgel used in this invention is as follows: (1) Soak the bacterial cellulose block in deionized water for cleaning to remove all impurities and electrolytes inside and outside until the TDS of the water after cleaning is 0 - 1 ppm. (2) Add the washed cubic bacterial cellulose block to 100 times its mass of deionized water, mix and then put it into a colloid mill. Adjust the gap of the colloid mill to 1 mm and grind it at the highest speed for 20 - 30 min. Finally, concentrate it using a rotary evaporator at 70 °C to obtain a cellulose microgel (CM) with a solid content of not less than 0.5%.

[0019] Figure 1 Fig. (A) is the particle size distribution diagram of the prepared cellulose microgel, Fig. (B) is the optical photo, and Fig. (C) is the structural micrograph. After measurement, the average particle size of its dispersion is 63 μm. The microstructure of CM is characterized by a field emission scanning electron microscope (FE - SEM). The results show that CM presents a nanoporous network structure constructed by cellulose nanofibers. Thus, CM exhibits the unique multi - scale structural characteristics of cellulose fibers, that is, the particles are at the micron scale and the internal pores are at the nanoscale, as Figure 2 shown in Fig. A. The micron - scale CM enables it to have good micron - scale dispersion performance in the water system.

[0020] Example 1

[0021] Weigh 100 grams of concentrated natural rubber latex (NRL) with a rubber content of about 60%, add 3 grams of bacterial cellulose microgel (CM) with a solid content of 0.5%, and stir it at 80 rpm for 20 min using mechanical stirring. The physical photo of the mixed system is as Figure 2 shown in Fig. B. The latex is evenly dispersed. Adding CM does not affect its appearance performance, and there is no problem of latex instability and sedimentation. The sample is denoted as CM / NRL - 1 and stored in a 4 °C refrigerator for later use.

[0022] Example 2

[0023] Weigh 100 grams of concentrated natural rubber latex (NRL) with a rubber content of about 60%, add 6 grams of bacterial cellulose microgel (CM) with a solid content of 0.5%, and stir it at 80 rpm for 20 min using mechanical stirring. As Figure 2It can be seen that in the mixed system, the latex shows a uniform dispersion state and no stratification phenomenon occurs. The sample is denoted as CM / NRL-2 and stored in a refrigerator at 4 °C for later use.

[0024] Example 3

[0025] Weigh 100 g of concentrated natural rubber latex (NRL) with a rubber content of about 60%, add 9 g of bacterial cellulose microgel (CM) with a solid content of 0.5% to it, and stir the mixture for 20 min under the condition of 80 rpm using mechanical stirring. The mixed system is as Figure 2 shown in Figure D. The latex is evenly dispersed and no problem of latex instability and sedimentation occurs. The sample is denoted as CM / NRL-3 and stored in a refrigerator at 4 °C for later use.

[0026] Example 4

[0027] Weigh 100 g of concentrated natural rubber latex (NRL) with a rubber content of about 60%, add 12 g of bacterial cellulose microgel (CM) with a solid content of 0.5% to it, and stir the mixture for 20 min under the condition of 80 rpm using mechanical stirring. The sample is denoted as CM / NRL-4 and stored in a refrigerator at 4 °C for later use. The physical picture is shown in Figure 2 Figure F.

[0028] Comparative Example 1 Weigh 100 g of concentrated natural rubber latex (NRL) with a rubber content of 60%, and stir it for 20 min under the condition of 80 rpm using mechanical stirring. The sample is denoted as NRL and stored in a refrigerator at 4 °C for later use. The physical picture is shown in Figure 2 Figure A.

[0029] The products of the above examples and comparative examples were subjected to rheological property detection, and the detection data are shown in Table 1 below.

[0030] Table 1

[0031] It can be seen from the rheological test data in Table 1 above that: (1) Viscosity characteristics: At 25 °C, a flow ramp test (shear rate 0.01 - 1000 s⁻¹, lasting 300 s) was carried out on each latex fluid to obtain an unsteady flow curve. The relationship between the system viscosity and the shear rate is shown in Figure 3, the specific numerical change rules are shown in Table 1. It was found that adding different concentrations of cellulose microgel (CM) to natural rubber latex (NRL) had different effects on the viscosity of the system. All samples showed the characteristics that as the shear rate increased, the viscosity first increased slightly and then continued to decrease. At low shear rates, the latex rubber particles were relatively stationary, and a network structure was formed through hydrogen bonds, van der Waals forces, and physical entanglement of phospholipid and protein molecular chains, increasing the flow resistance and resulting in an increase in viscosity. As the shear rate increased, the network structure was destroyed, the flow resistance decreased, and the fluid showed shear thinning. Among them, the higher the CM content, the more significant the shear thinning behavior. Taking CM / NRL-4 as an example, its viscosity decreased from about 5.80 Pa·s at low shear rates to about 0.02 Pa·s at high shear rates. This is because under high shear, the self-network structure of CM ( Figure 1 C) was disentangled from the latex particles, weakening the hindrance to fluid flow. When the shear rate of CM / NRL-4 was greater than 100 s⁻¹, the viscosity was the lowest; the viscosity of CM / NRL-1 was the highest at this time, but the viscosities of both were lower than that of NRL without adding CM. This result confirmed that CM could effectively change the rheological properties of latex fluid.

[0032] (2) Yield stress: The yield value is the minimum shear stress required to be applied before the material starts to flow or plastically deform. Under this stress, the material maintains its shape in a solid state and shows elasticity under small stresses; after the stress exceeds the yield value, the material shows the viscous flow characteristics of a liquid state. At 25 °C, a flow ramp test (shear rate 0.01 - 1000 s⁻¹, test duration 300 s) was carried out on NRL and CM / NRL. According to Figure 4 and the data in Table 1, it showed that: NRL had no obvious yield behavior, CM / NRL-2 had weak yield characteristics, while CM / NRL-3 and CM / NRL-4 showed significant yield characteristics. The yield stress of NRL was the lowest, only 0.045 Pa, lower than all samples added with CM. This was because the polyisoprene long-chain polymer in natural rubber latex was easy to stretch and slide under pressure. It can be seen that adding CM to natural latex can effectively improve the fluid stability.

[0033] (3) Thixotropy: The thixotropy loop test is a method based on rheology to evaluate the thixotropic properties of materials. The test was carried out at 25 °C and scanned twice: the first time the shear rate was increased from 0.001 s⁻¹ to 1000 s⁻¹, and the second time from 1000 s⁻¹ to 0.001 s⁻¹. The viscosity-shear rate closed curve formed during the increase and decrease of the shear rate of the material is the thixotropy loop, and the area inside the loop can quantify the thixotropy degree. The larger the area, the stronger the thixotropy. The test results of NRL and CM / NRL ( Figure 5 , Table 1) showed that: NRL ( Figure 5A) The viscosity first increases and then decreases with the shear rate, no thixotropic loop is formed and the viscosity after shearing cannot be fully restored, indicating that it lacks elastic support and has no thixotropic property; adding 0.025 g CM of NRL ( Figure 5 B) There is also no thixotropic loop, indicating that the interaction between CM and NR particles is weak at this dosage, but the shear recovery performance is improved; when the CM addition amount reaches 0.05 - 0.2 g ( Figure 5 C, D, E), an obvious thixotropic loop appears in NRL, confirming that sufficient CM can endow NRL with thixotropy. The complete closed thixotropic loop indicates that its thixotropic behavior has good reversibility, which is beneficial to the mold infiltration and anti-dripping in the preparation of impregnated products.

[0034] (4) Test on the structure recovery performance after oscillatory shear: At a fixed frequency, by switching between small-amplitude oscillatory shear and large-amplitude oscillatory shear, the performance change laws such as viscoelasticity and rheological properties of NRL and CM / NR during the process of passing from the linear viscoelastic region through the non-linear viscoelastic region and then returning to the linear viscoelastic region are systematically explored. During the test, in the first 100 s low-strain scanning stage, the storage modulus of all samples is higher than the loss modulus, showing elastic characteristics; in the 100 - 200 s high-strain scanning stage, the oscillatory strain exceeds the linear viscoelastic region, and the loss modulus exceeds the storage modulus, and the samples show viscosity; in the 200 - 600 s, the low oscillatory strain test is restored to evaluate the recovery ability of the samples.

[0035] As Figure 6 shown, in the low-strain scanning stage after 200 s of NRL, although the storage modulus slowly increases, it still does not return to the initial value until the end of the test, indicating that the large-strain scanning causes irreversible damage to its colloidal structure, or the chain structure relaxation time is too long. On the contrary, the CM / NRL samples can gradually return to the initial state after the large-amplitude oscillatory strain test. Among them, CM / NRL-4 performs particularly well, and the storage modulus is restored within about 5 s. This is attributed to the "elasticity" of CM network structure endowing NRL, effectively resisting the irreversible damage of the structure under large-amplitude oscillatory strain. This conclusion echoes the previous research results of thixotropy and yield value. This property is of great significance in the actual impregnation process, which can prompt the natural latex after impregnation to quickly recover after coagulation and molding, providing key support for optimizing the molding process.

[0036] Example 5 (1) Sulfur powder, zinc oxide and zinc dibutyldithiocarbamate powder are prone to agglomeration problems during the preparation of latex impregnated products. Therefore, they are usually added in the form of dispersions during the pre-vulcanization process. The specific preparation method is as follows: ① Sulfur dispersion: After weighing 1.0 g of sulfur powder, 0.76 g of a 10% casein solution (prepared by dissolving 1.0 g of casein in 9.0 g of deionized water through ultrasonic treatment for 5 min and stirring for 10 min), 0.1 g of a 10% potassium hydroxide solution (prepared by dissolving 1.0 g of potassium hydroxide in 9.0 g of deionized water), and 1.0 g of deionized water were added in sequence. Subsequently, ultrasonic treatment was carried out for 5 min and stirring was carried out at 500 rpm for 30 min to obtain a sulfur dispersion system; ② Zinc oxide dispersion: After weighing 1.0 g of zinc oxide, 0.76 g of a 10% casein solution (prepared by dissolving 1.0 g of casein in 9.0 g of deionized water through ultrasonic treatment for 5 min and stirring for 10 min), 0.1 g of a 10% potassium hydroxide solution (prepared by dissolving 1.0 g of potassium hydroxide in 9.0 g of deionized water), and 1.0 g of deionized water were added in sequence. Subsequently, ultrasonic treatment was carried out for 5 min and stirring was carried out at 500 rpm for 30 min to obtain a zinc oxide dispersion system; ③ Sodium methylenebis(naphthalenesulfonate) dispersion: After weighing 1.0 g of sodium methylenebis(naphthalenesulfonate), 0.76 g of a 10% casein solution (prepared by dissolving 1.0 g of casein in 9.0 g of deionized water through ultrasonic treatment for 5 min and stirring for 10 min), 0.1 g of a 10% potassium hydroxide solution (prepared by dissolving 1.0 g of potassium hydroxide in 9.0 g of deionized water), and 1.0 g of deionized water were added in sequence. Subsequently, ultrasonic treatment was carried out for 5 min and stirring was carried out at 500 rpm for 30 min to obtain a sodium methylenebis(naphthalenesulfonate) dispersion system; (2) Weigh 100 g of concentrated natural rubber latex (NRL) with a rubber content of about 60%. First, add 3 g of CM with a solid content of 0.5% to it, and then weigh 6 g, 9 g, and 3 g of the sulfur, zinc oxide, and zinc dibutyldithiocarbamate dispersions prepared in advance in step (1) and add them respectively. After the feeding is completed, the temperature is raised to 50 °C, and mechanical stirring is carried out at 80 rpm for 20 min to obtain an NR mixed solution; (3) Immerse a rectangular tempered glass substrate in a coagulant solution (the coagulant formula is calcium nitrate with a mass fraction of 50%, Triton with a mass fraction of 0.06%, and deionized water) for 10 s. Subsequently, the glass substrate coated with the coagulant is dried at 65 °C and cooled to room temperature; (4) Slowly immerse the tempered glass treated in step (3) into the NR mixed solution prepared in advance in step (2) and let it stand for 1 min. Slowly take out the impregnated glass substrate from the impregnation bottle and cure it at 70 °C in a convection oven for 1 hour to form a composite latex film. The sample is denoted as CM / NR-1.

[0037] Example 6 (1) Sulfur powder, zinc oxide, and zinc dibutyldithiocarbamate powder are prone to agglomeration problems during the preparation of latex impregnated products. Therefore, they are usually added in the form of dispersions during the pre-vulcanization process. The specific preparation methods are as follows: ① Sulfur dispersion: After weighing 1.0 g of sulfur powder, 0.76 g of a 10% casein solution (prepared by dissolving 1.0 g of casein in 9.0 g of deionized water through ultrasonic treatment for 5 min and stirring for 10 min), 0.1 g of a 10% potassium hydroxide solution (prepared by dissolving 1.0 g of potassium hydroxide in 9.0 g of deionized water), and 1.0 g of deionized water are added in sequence. Subsequently, ultrasonic treatment is carried out for 5 min and stirring is carried out at 500 rpm for 30 min to obtain a sulfur dispersion system; ② Zinc oxide dispersion: After weighing 1.0 g of zinc oxide, 0.76 g of a 10% casein solution (prepared by dissolving 1.0 g of casein in 9.0 g of deionized water through ultrasonic treatment for 5 min and stirring for 10 min), 0.1 g of a 10% potassium hydroxide solution (prepared by dissolving 1.0 g of potassium hydroxide in 9.0 g of deionized water), and 1.0 g of deionized water are added in sequence. Subsequently, ultrasonic treatment is carried out for 5 min and stirring is carried out at 500 rpm for 30 min to obtain a zinc oxide dispersion system; ③ Sodium methylene bisnaphthalenesulfonate dispersion: After weighing 1.0 g of sodium methylene bisnaphthalenesulfonate, 0.76 g of a 10% casein solution (prepared by dissolving 1.0 g of casein in 9.0 g of deionized water through ultrasonic treatment for 5 min and stirring for 10 min), 0.1 g of a 10% potassium hydroxide solution (prepared by dissolving 1.0 g of potassium hydroxide in 9.0 g of deionized water), and 1.0 g of deionized water are added in sequence. Subsequently, ultrasonic treatment is carried out for 5 min and stirring is carried out at 500 rpm for 30 min to obtain a sodium methylene bisnaphthalenesulfonate dispersion system; (2) Weigh 100 g of concentrated natural rubber latex (NRL) with a rubber content of approximately 60%. First, add 6 g of CM with a solid content of 0.5% to it, and then weigh 6 g, 9 g, and 3 g of the sulfur, zinc oxide, and zinc dibutyldithiocarbamate dispersions prepared in step (1) respectively and add them. After the feeding is completed, the temperature is raised to 50 °C, and mechanical stirring is carried out at 80 rpm for 20 min to obtain an NR mixture; (3) Immerse a rectangular tempered glass substrate in a coagulant solution (the coagulant formula is 50% calcium nitrate by mass fraction, 0.06% Triton by mass fraction, and deionized water) for 10 seconds. Subsequently, dry the glass substrate coated with the coagulant at 65 °C and cool it to room temperature; (4) Slowly immerse the tempered glass processed in step (3) into the NR mixture prepared in advance in step (2) and let it stand for 1 min. Slowly take out the impregnated glass substrate from the impregnation bottle and cure it in a convection oven at 70 °C for 1 hour to form a composite latex film. The sample is denoted as CM / NR-2.

[0038] Example 7 (1) Sulfur powder, zinc oxide, and zinc dibutyldithiocarbamate powder are prone to agglomeration problems during the preparation of latex impregnated products. Therefore, they are usually added in the form of dispersions during the pre-vulcanization process. The specific preparation method is as follows: ① Sulfur dispersion: After weighing 1.0 g of sulfur powder, sequentially add 0.76 g of a 10% casein solution (prepared by dissolving 1.0 g of casein in 9.0 g of deionized water by ultrasonic treatment for 5 min and stirring for 10 min), 0.1 g of a 10% potassium hydroxide solution (prepared by dissolving 1.0 g of potassium hydroxide in 9.0 g of deionized water), and 1.0 g of deionized water. Then, perform ultrasonic treatment for 5 min and stir at 500 rpm for 30 min to obtain a sulfur dispersion system; ② Zinc oxide dispersion: After weighing 1.0 g of zinc oxide, sequentially add 0.76 g of a 10% casein solution (prepared by dissolving 1.0 g of casein in 9.0 g of deionized water by ultrasonic treatment for 5 min and stirring for 10 min), 0.1 g of a 10% potassium hydroxide solution (prepared by dissolving 1.0 g of potassium hydroxide in 9.0 g of deionized water), and 1.0 g of deionized water. Then, perform ultrasonic treatment for 5 min and stir at 500 rpm for 30 min to obtain a zinc oxide dispersion system; ③ Sodium methylenebis(naphthalenesulfonate) dispersion: After weighing 1.0 g of sodium methylenebis(naphthalenesulfonate), sequentially add 0.76 g of a 10% casein solution (prepared by dissolving 1.0 g of casein in 9.0 g of deionized water by ultrasonic treatment for 5 min and stirring for 10 min), 0.1 g of a 10% potassium hydroxide solution (prepared by dissolving 1.0 g of potassium hydroxide in 9.0 g of deionized water), and 1.0 g of deionized water. Then, perform ultrasonic treatment for 5 min and stir at 500 rpm for 30 min to obtain a sodium methylenebis(naphthalenesulfonate) dispersion system; (2) Weigh 100 g of concentrated natural rubber latex (NRL) with a rubber content of about 60%. First, add 9 g of CM with a solid content of 0.5% to it. Then, separately weigh 6 g, 9 g, and 3 g of the sulfur, zinc oxide, and zinc dibutyldithiocarbamate dispersions prepared in advance in step (1) and add them. After the addition is completed, raise the temperature to 50 °C and stir with a mechanical stirrer at 80 rpm for 20 min to obtain an NR mixture; (3) Immerse the rectangular tempered glass substrate in the coagulant solution (the coagulant formula is calcium nitrate with a mass fraction of 50%, Triton with a mass fraction of 0.06%, and deionized water) for 10 seconds, and then dry the glass substrate coated with the coagulant at 65 °C and cool it to room temperature; (4) Slowly immerse the tempered glass treated in step (3) into the NR mixture prepared in advance in step (2) and let it stand for 1 min. Slowly take out the impregnated glass substrate from the impregnation bottle and cure it in a convection oven at 70 °C for 1 hour to form a composite latex film. The sample is denoted as CM / NR-3.

[0039] Example 8 (1) Sulfur powder, zinc oxide, and zinc dibutyldithiocarbamate powder are prone to agglomeration problems during the preparation of latex impregnated products. Therefore, they are usually added in the form of a dispersion during the pre-vulcanization process. The specific preparation method is as follows: ① Sulfur dispersion: After weighing 1.0 g of sulfur powder, sequentially add 0.76 g of a 10% casein solution (prepared by dissolving 1.0 g of casein in 9.0 g of deionized water by ultrasonic treatment for 5 min and stirring for 10 min), 0.1 g of a 10% potassium hydroxide solution (prepared by dissolving 1.0 g of potassium hydroxide in 9.0 g of deionized water), and 1.0 g of deionized water. Then, perform ultrasonic treatment for 5 min and stir at 500 rpm for 30 min to obtain a sulfur dispersion system; ② Zinc oxide dispersion: After weighing 1.0 g of zinc oxide, sequentially add 0.76 g of a 10% casein solution (prepared by dissolving 1.0 g of casein in 9.0 g of deionized water by ultrasonic treatment for 5 min and stirring for 10 min), 0.1 g of a 10% potassium hydroxide solution (prepared by dissolving 1.0 g of potassium hydroxide in 9.0 g of deionized water), and 1.0 g of deionized water. Then, perform ultrasonic treatment for 5 min and stir at 500 rpm for 30 min to obtain a zinc oxide dispersion system; ③ Sodium methylene bisnaphthalenesulfonate dispersion: After weighing 1.0 g of sodium methylene bisnaphthalenesulfonate, sequentially add 0.76 g of a 10% casein solution (prepared by dissolving 1.0 g of casein in 9.0 g of deionized water by ultrasonic treatment for 5 min and stirring for 10 min), 0.1 g of a 10% potassium hydroxide solution (prepared by dissolving 1.0 g of potassium hydroxide in 9.0 g of deionized water), and 1.0 g of deionized water. Then, perform ultrasonic treatment for 5 min and stir at 500 rpm for 30 min to obtain a sodium methylene bisnaphthalenesulfonate dispersion system; (2)Weigh 100 g of concentrated natural rubber latex (NRL) with a rubber content of approximately 60%. First, add 12 g of CM with a solid content of 0.5% to it. Then, weigh 6 g, 9 g, and 3 g of the sulfur, zinc oxide, and zinc dibutyldithiocarbamate dispersions prepared in advance in step (1) respectively and add them. After the feeding is completed, raise the temperature to 50 °C and stir for 20 min at 80 rpm using mechanical stirring to obtain an NR mixture; (3)Immerse a rectangular tempered glass substrate in a coagulant solution (the coagulant formula is calcium nitrate with a mass fraction of 50%, Triton with a mass fraction of 0.06%, and deionized water) for 10 seconds. Subsequently, dry the glass substrate coated with the coagulant at 65 °C and cool it to room temperature; (4)Slowly immerse the tempered glass treated in step (3) into the NR mixture prepared in advance in step (2) and let it stand for 1 min. Slowly take out the impregnated glass substrate from the impregnation bottle and cure it at 70 °C in a convection oven for 1 hour to form a composite latex film. The sample is denoted as CM / NR-4.

[0040] Comparative Example 2 (1)Sulfur powder, zinc oxide, and zinc dibutyldithiocarbamate powder are prone to agglomeration problems during the preparation of latex impregnated products. Therefore, they are usually added in the form of dispersions during the pre-vulcanization process. The specific preparation method is as follows: ① Sulfur dispersion: After weighing 1.0 g of sulfur powder, successively add 0.76 g of a 10% casein solution (prepared by dissolving 1.0 g of casein in 9.0 g of deionized water through ultrasonic treatment for 5 min and stirring for 10 min), 0.1 g of a 10% potassium hydroxide solution (prepared by dissolving 1.0 g of potassium hydroxide in 9.0 g of deionized water), and 1.0 g of deionized water. Subsequently, perform ultrasonic treatment for 5 min and stir at 500 rpm for 30 min to obtain a sulfur dispersion system; ② Zinc oxide dispersion: After weighing 1.0 g of zinc oxide, successively add 0.76 g of a 10% casein solution (prepared by dissolving 1.0 g of casein in 9.0 g of deionized water through ultrasonic treatment for 5 min and stirring for 10 min), 0.1 g of a 10% potassium hydroxide solution (prepared by dissolving 1.0 g of potassium hydroxide in 9.0 g of deionized water), and 1.0 g of deionized water. Subsequently, perform ultrasonic treatment for 5 min and stir at 500 rpm for 30 min to obtain a zinc oxide dispersion system; ③ Sodium methylene bisnaphthalene sulfonate dispersion: After weighing 1.0 g of sodium methylene bisnaphthalene sulfonate, 0.76 g of a 10% casein solution (prepared by dissolving 1.0 g of casein in 9.0 g of deionized water through 5 min of ultrasonic treatment and 10 min of stirring), 0.1 g of a 10% potassium hydroxide solution (prepared by dissolving 1.0 g of potassium hydroxide in 9.0 g of deionized water), and 1.0 g of deionized water were added in sequence. Subsequently, ultrasonic treatment was carried out for 5 min and stirring was carried out at 500 rpm for 30 min to obtain a sodium methylene bisnaphthalene sulfonate dispersion system; (2) Weigh 100 g of concentrated natural rubber latex (NRL) with a rubber content of about 60%. Add 6 g of sulfur dispersion, 9 g of zinc oxide dispersion, and 3 g of zinc dibutyldithiocarbamate dispersion thereto, then heat up to 50 °C, and use mechanical stirring to stir at 80 rpm for 20 min to obtain an NR mixture; (3) Immerse a rectangular tempered glass substrate in a coagulant solution (the coagulant formula is 50% calcium nitrate by mass fraction, 0.06% Triton by mass fraction, and deionized water) for 10 seconds. Subsequently, dry the glass substrate coated with the coagulant at 65 °C and cool it to room temperature; (4) Slowly immerse the tempered glass treated in step (3) into the NR mixture prepared in advance in step (2) and let it stand for 1 min. Slowly take out the impregnated glass substrate from the impregnation bottle, and cure it at 70 °C in a convection oven for 1 hour to form a composite latex film, and the sample is denoted as NR.

[0041] The mechanical tensile test results of the natural rubber latex and cellulose composite system impregnated films prepared in Examples 5 - 8 and Comparative Example 2 of the present invention are shown in Table 2 below: Table 2 Serial number Sample <![CDATA[Toughness (KJ / cm 3 )]]> Elongation at break (%) Tensile strength (MPa) <![CDATA[Energy consumption difference between the first and the last in 5 cycles of 30% strain cyclic tension (KJ / cm 3 ) <!-- 8 -->]]> Comparative example 2 NR 15.06 1019 4.79 2.00 Example 5 CM / NR-1 19.89 1082 6.37 2.32 Example 6 CM / NR-2 34.50 1259 12.59 2.57 Example 7 CM / NR-3 21.70 852 6.73 4.65 Example 8 CM / NR-4 18.60 942 5.41 2.62 Figure 7 A shows the stress - strain curves of NR and CM / NR nanocomposites. Combining Figure 7 with the data analysis in Table 2 above, the results show that the tensile strength of the CM / NR nanocomposite film is significantly higher than that of the NR film and increases with the increase of the cellulose content. The specific values are as follows: (1) Toughness: From the data, the toughness of the natural rubber film (NR, Comparative Example 2) without adding cellulose microgel (CM) is 15.06 / (KJ / cm 3 ). After adding CM, the toughness of the CM / NR - 1 composite film (Example 5) is increased to 19.89 / (KJ / cm 3 ), and CM / NR - 2 (Example 6) reaches 34.50 / (KJ / cm 3 ), indicating that adding different amounts of CM can improve its toughness to varying degrees.

[0042] (2) Elongation at break: The elongation at break of the NR film is 1016%. After adding CM, the elongation at break of CM / NR-1 reaches 1088%, and that of CM / NR-2 is 1259%. The ductility of the material is significantly improved.

[0043] (3) Tensile strength: The tensile strength of the NR film is 4.79 MPa. After adding CM, the tensile strength of CM / NR-1 increases to 6.37 MPa, and that of CM / NR-2 reaches 12.59 MPa.

[0044] (4) Energy consumption difference between the first and last stretching cycles of 5 times with a strain of 30%: The energy consumption difference of NR is 2.00 / (KJ / cm 3 ), CM / NR-1 is 2.32 / (KJ / cm 3 ), CM / NR-2 is 2.57 / (KJ / cm 3 ).

[0045] In addition, if Figure 7 CM / NR-3 and 4 shown in C also show a decrease in toughness and elongation at break. This is because the chemical structure of NR particles - polyisoprene is not compatible with cellulose. Excessive addition of CM can easily cause aggregation and lead to stress concentration during the stretching process, resulting in changes in mechanical properties. Therefore, only a moderate amount of CM addition has the most significant effect on the mechanical toughening of the film.

[0046] Comprehensive data show that cellulose microgel (CM) toughened rubber film has significant advantages, and the key to its toughening mechanism lies in: the micron-scale network structure of CM is composed of interwoven cellulose nanofibers, which are both stable and flexible. When subjected to force, the network elastically deforms and stores energy in the initial stretching; after the strain increases, the energy is further dissipated through plastic deformation such as stretching and twisting; when the strain is large enough, part of the network "sacrifices" to break, absorbing a large amount of energy to prevent crack propagation, greatly improving the toughness of the material. In addition, the "soft" properties of CM enable it to deform synergistically with the NR matrix, disperse stress, and avoid stress concentration, which has a positive effect on improving elongation at break and tensile strength.

[0047] Thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) tests were performed on the natural latex films and cellulose composite system impregnated films prepared in Examples 5 to 8 and Comparative Example 2. Figure 8 It can be seen that: (1) By Figure 8 It can be seen from A that all samples begin to thermally degrade at around 300 °C and end at around 450 °C, and with the increase of CM filler content, the final residual mass increases accordingly and the thermal stability improves.

[0048] (2) Figure 8Figure B shows the results of differential scanning calorimetry (DSC) tests on the glass transition temperature (Tg) of natural rubber composite films. The glass transition temperature increases with the increase in the filler content because the network structure of CM restricts the migration of polyisoprene segments, reduces the free volume of the rubber, and thus leads to an increase in the glass transition temperature.

[0049] In summary, due to its micron-scale network structure, when CM interacts with the NR matrix, it can not only play a thermal barrier role in thermal decomposition to increase the maximum thermal decomposition temperature but also improve the glass transition temperature by restricting the movement of molecular segments. It shows a unique advantage in improving the thermal stability of rubber films, being significantly superior to pure NR films without CM addition.

[0050] Figure 9 The cross-sectional microstructure of CM / NR-3 is presented. CM with an average particle size of about 10 μm is dispersed in the NR matrix and has a certain degree of entanglement with the NR matrix. However, due to the hydrophobicity of the NR matrix, the bonding force at the interface between CM and NR is weak. It can be seen from the EDS energy spectrum analysis that S and Zn, as processing aids, can be evenly dispersed in NR.

[0051] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the foregoing embodiments or perform equivalent replacements for some of the technical features. These modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for improving the stability and toughness of natural latex, characterized in that, Prepare bacterial cellulose into cellulose microgels, add the cellulose microgels into natural rubber latex, realize the stable preservation of low-ammonia natural rubber latex, and improve the toughness.

2. The method for improving the stability and toughness of natural latex according to claim 1, wherein The preparation method of the cellulose microgels is specifically as follows: (1) Immerse the bacterial cellulose block in deionized water for soaking and cleaning until the TDS of the water after cleaning is 0 - 1 ppm; (2) Add the washed bacterial cellulose block into deionized water more than 100 times its mass. After mixing, place it in a colloid mill, adjust the gap to 1 mm, and grind it at the highest speed for 20 - 30 minutes. Finally, use a rotary evaporator to concentrate at 70 °C to obtain a cellulose microgel solution with a solid content of not less than 0.5%.

3. The method for improving the stability and toughness of natural latex according to claim 1, wherein When adding the cellulose microgels into natural rubber latex, based on the dry weight of the natural rubber latex, the added mass fraction of the cellulose microgels is 0.025 - 0.2%.

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