Natural rubber with high conductivity and preparation method thereof

Abstract

The invention discloses natural rubber with high conductivity and a preparation method thereof, and belongs to the technical field of conductive rubber.The natural rubber is prepared from 100 parts of natural latex, 5-40 parts of conductive filler, 0.5-20 parts of cellulose, 1-10 parts of zinc oxide, 0.5-5 parts of sulfur, 0.5-5 parts of accelerant, 1-5 parts of stearic acid, 0.05-1 part of potassium hydroxide, 0.05-1 part of dispersing agent NF and 0.5-20 parts of casein; the natural rubber with high conductivity is obtained through preparation of a natural latex solution, cellulose treatment and preparation of a cellulose solution, mixing of a conductive filler and the cellulose solution to prepare a suspension, mixing of the suspension and the natural latex, solidification forming by a low-temperature freezing method, and hot-pressing vulcanization. The method has the advantages of being simple in process, low in cost, environmentally friendly and the like, and the produced composite material has good electrical properties, flexibility and mechanical properties.

Description

Technical Field

[0001] The invention relates to the technical field of conductive rubber, and in particular to a natural rubber with high conductive performance and a preparation method thereof. Background Art

[0002] In recent years, flexible electronic devices have attracted widespread attention in human-computer interaction, wearable devices, and smart sensors due to their excellent flexibility and stretchability. Among them, natural rubber is a good flexible conductive material matrix due to its excellent high elasticity, flexibility, and stretchability. As a new type of material, conductive natural rubber has significant advantages such as good elasticity, strong wear resistance, excellent processability, light weight, and environmental protection. Compared with traditional metal conductive materials, it has shown unique advantages in many fields and has been widely used in electronics, automobiles, aerospace, medical and wearable devices. Pure natural rubber does not have electrical conductivity, and conductive fillers need to be added thereto. The most commonly used conductive fillers added to pure natural rubber are graphite, graphene and carbon nanotubes. Generally, conductive natural rubber composite materials can be prepared by mixing natural rubber with a single conductive filler, but a higher content is often required to meet the conductivity requirements. In addition, due to the physical properties of the nanofiller itself, it is very easy to agglomerate in the natural rubber matrix, which limits the conductive properties and physical and mechanical properties of the conductive natural rubber composite materials. In general, the current conductive natural rubber has problems in the preparation process such as poor dispersibility of conductive fillers, difficulty in balancing conductive properties and mechanical properties, influence of processing technology on conductive properties, and low conductivity.

[0003] Therefore, the object of the present invention is to provide a method for preparing high-performance conductive natural rubber to solve the problems raised in the above background technology. Summary of the invention

[0004] The invention aims to solve the problems of difficult dispersion of conductive fillers, poor mechanical properties and poor conductive properties in the existing conductive rubber preparation process, and provides a high-performance conductive natural rubber and a preparation method thereof. The method introduces cellulose, disperses the conductive fillers, and constructs a conductive network by utilizing the excellent aspect ratio and high specific surface area of ​​cellulose. A new low-temperature freezing method is then used to coagulate natural rubber latex, thereby improving the mechanical properties of the composite material. The method has the advantages of simple process, low cost, green environmental protection, etc., and the prepared composite material has good electrical properties, flexibility and mechanical properties.

[0005] In order to achieve these purposes and other advantages of the present invention, the present invention provides a method for preparing a natural rubber with high electrical conductivity, wherein the raw materials, measured in parts by weight, include: 100 parts of natural rubber latex, 5-40 parts of conductive filler, 0.5-20 parts of cellulose, 1-10 parts of zinc oxide, 0.5-5 parts of sulfur, 0.5-5 parts of accelerator, 1-5 parts of stearic acid, 0.05-1 parts of potassium hydroxide, 0.05-1 parts of diffusant NF, 0.5-20 parts of casein; The following steps are involved: Step 1, preparation of natural rubber latex solution: weigh natural rubber latex and prepare a solution of a certain concentration, and then remove ammonia from the natural rubber latex by formaldehyde method; Add casein, dispersant NF and potassium hydroxide into water to prepare a solution with a concentration of 10-40%; place zinc oxide, sulfur, accelerator M and stearic acid together with casein solution, dispersant NF solution and potassium hydroxide solution in a ball mill for grinding at a speed of 500-3500 rpm and a ball milling time of 2-72 h to obtain a dispersion; add the dispersion into natural rubber latex after ammonia removal treatment, stir for a certain period of time, and then ripen at room temperature for 0.5-6 h at a stirring speed of 100-500 rpm to obtain a prepared natural rubber latex solution; Step 2, treating cellulose and preparing a cellulose solution with a concentration of 0.5-5%; Step 3, the conductive filler is mixed with the cellulose solution to form a suspension: the conductive filler is added to the cellulose solution obtained in step 2, and dispersed evenly under high-speed stirring at a stirring speed of 500 to 3000 rpm for a stirring time of 0.5 to 4 hours to obtain a suspension; Step 4, mixing the suspension with the natural latex solution: adding casein to water to prepare a solution with a concentration of 10-40%, and adding the solution to the suspension obtained in step 3, stirring evenly, and then adding the prepared natural latex solution, stirring at room temperature for 0.5-6 h, with a stirring speed of 50-500 rpm, to obtain a mixed solution; Step 5, low temperature freezing method solidification molding: pour the mixed solution obtained in step 4 into a mold, and freeze it on a low temperature cold source for 1 to 48 hours, and the freezing temperature is -20°C to -80°C; Step 6, hot pressing vulcanization: thawing the frozen material obtained in step 5 at room temperature, pre-pressing for 1 to 5 minutes at a pressure of 1 to 5 MPa for multiple times after thawing; drying the pre-pressed material in a 20 to 80° C. forced air drying oven for 3 to 50 h; placing the material on a vulcanizer for hot pressing vulcanization after drying, the vulcanization pressure is 5 to 15 MPa, the hot pressing temperature is 120 to 145° C., and the hot pressing time is 10 to 30 minutes. After the hot pressing is completed, a conductive natural rubber film is obtained.

[0006] Or it can be formed into other shapes according to production needs, including but not limited to films.

[0007] Preferably, the drying method is one of natural air drying, forced air drying and freeze drying.

[0008] Preferably, during the low-temperature freezing coagulation molding process of step 5, a two-step freezing treatment method is adopted. First, a cold source is applied in a specific direction of the mixed solution, and the cellulose-conductive filler network and natural latex are repelled and squeezed by the growth of ice crystals, gradually approach and gather in the gaps between the ice crystals to arrange and form; then further uniform freezing treatment is performed, so that the natural latex in the molded structure is coagulated and fixed at low temperature to form a pre-cross-linked network. When the water is removed, the network skeleton left behind will completely replicate the cellular structure of the ice crystals (regularly arranged polyhedral structure), thereby realizing the arrangement and distribution of the cellulose-conductive filler network and improving the comprehensive performance of the obtained natural rubber composite film.

[0009] Specifically, the freezing technology includes but is not limited to one of the freezing methods such as unidirectional freezing, bidirectional freezing, homogeneous freezing, radial freezing, gradient freezing, etc.

[0010] Preferably, the cold source temperature used for freezing is -20°C to -80°C.

[0011] Preferably, the conductive filler is any one or more of carbon-based conductive fillers such as graphite, graphene, and carbon nanotubes.

[0012] Preferably, the cellulose is any one or more of natural cellulose, modified cellulose, nanocellulose, etc., wherein the length of the fiber is ≤5 mm and the diameter is ≤20 μm.

[0013] Preferably, the concentration of natural rubber latex is 20-65%.

[0014] Preferably, the accelerator is one or more of accelerator M, accelerator ZDC, accelerator DM and accelerator DC.

[0015] Using formaldehyde to remove ammonia from natural rubber latex is a common method. The basic principle is to remove ammonia from natural rubber latex through a chemical reaction between formaldehyde and ammonia.

[0016] The present invention provides a natural rubber with high electrical conductivity obtained by the above preparation method.

[0017] The present invention has at least the following beneficial effects: 1. The natural rubber with high conductivity of the present invention is added with cellulose, and cellulose is used as a dispersion carrier of the conductive filler. The growth of ice crystals during the freezing process is used as a dispersion means for the cellulose-conductive filler network, thereby avoiding dispersion problems such as sedimentation and agglomeration caused by the conductive filler due to its own high density and high surface energy. At the same time, cellulose has a high aspect ratio and a high specific surface area, which can better disperse the conductive filler, and the cellulose loaded with the conductive filler can also play a role in constructing a conductive network, thereby improving the performance of the obtained natural rubber composite film.

[0018] 2. The present invention utilizes a low-temperature freezing coagulation method to coagulate the natural latex mixed solution, and does not use other coagulation methods such as acid coagulation, which is more environmentally friendly, and the obtained natural rubber composite material has better mechanical properties.

[0019] 3. The freezing treatment method of the present invention is divided into two steps. First, a cold source is applied in a specific direction of the mixed solution, and the cellulose-conductive filler network and natural latex are repelled and squeezed by the growth of ice crystals, gradually approaching and gathering in the gaps between ice crystals to arrange and form; then further freezing treatment is performed, so that the natural latex in the molded structure is solidified at low temperature to form a pre-cross-linked network. When the water is removed, the remaining network skeleton will completely replicate the cellular structure of the ice crystals (regularly arranged polyhedral structure), thereby realizing the arrangement and distribution of the cellulose-conductive filler network and improving the comprehensive performance of the obtained natural rubber composite film.

[0020] 4. The method of the present invention has the advantages of simple process and low cost, and can be used for industrial promotion.

[0021] Other advantages, objectives and features of the present invention will be embodied in part through the following description, and in part will be understood by those skilled in the art through study and practice of the present invention. DETAILED DESCRIPTION

[0022] The present invention is further described in detail below in conjunction with embodiments so that those skilled in the art can implement the invention with reference to the description.

[0023] Example 1-1 This embodiment provides an example of a natural rubber with high electrical conductivity and a preparation method thereof.

[0024] The raw materials of this embodiment are formulated according to solid content as follows: 100 parts of natural rubber latex; 10 parts of graphite; 5 parts of pulp fiber; 3 parts of zinc oxide; 2 parts of sulfur; 1 part of accelerator M; 2 parts of stearic acid; 0.05 parts of potassium hydroxide; 0.05 parts of diffusant NF; and 0.5 parts of casein.

[0025] The preparation method comprises the following steps: Step 1, preparation of natural rubber latex solution: weigh natural rubber latex and prepare a solution of a certain concentration, and then remove ammonia from the natural rubber latex by formaldehyde method; Add casein, dispersant NF and potassium hydroxide into water to prepare a solution with a concentration of 10-40%; place zinc oxide, sulfur, accelerator M and stearic acid together with casein solution, dispersant NF solution and potassium hydroxide solution in a ball mill for grinding at a speed of 2500 rpm for 2 h to obtain a dispersion; add the dispersion into natural rubber latex after ammonia removal treatment, stir for a certain period of time and then ripen at room temperature for 2 h at a stirring speed of 500 rpm to obtain a prepared natural rubber latex solution; Step 2, weighing pulp fibers and adding water, and preparing a 2% concentration microfibrillated cellulose solution by means of electric stirring, ultrasonic dispersion or wall breaking; Step 3, graphite and microfibrillated cellulose solution are mixed to prepare graphite / cellulose suspension: graphite is added to the microfibrillated cellulose solution obtained in step 2 according to weight proportion, and dispersed evenly under high-speed stirring at a stirring speed of 1000 rpm for 1 h to obtain a graphite / microfibrillated cellulose suspension; Step 4, mixing the graphite / microfibrillated cellulose suspension with natural latex: adding casein to water to prepare a solution with a concentration of 20%, and adding it to the suspension obtained in step 3, stirring evenly, and then adding the prepared natural latex solution, stirring at room temperature for 0.5 h, the stirring speed is 200 rpm, to obtain a mixed solution; Step 5, low temperature freezing method solidification molding: pour the mixed solution obtained in step 4 into a mold and freeze it evenly in a -24°C cold box for 24 hours; Step 6, hot pressing vulcanization: thaw the frozen material obtained in step (5) at room temperature, and pre-press for 5 minutes at a pressure of 1 MPa multiple times after thawing; dry the pre-pressed material in a 60°C forced air drying oven for 8 hours; after drying, place the material on a vulcanizer for hot pressing vulcanization at a vulcanizing pressure of 10 MPa, a hot pressing temperature of 143°C, and a hot pressing time of 10 minutes to obtain a 2 mm thick conductive natural rubber film.

[0026] Example 1-2 This embodiment provides an example of a natural rubber with high electrical conductivity and a preparation method thereof.

[0027] The raw material ratio of this embodiment is as follows: 100 parts of natural rubber latex; 20 parts of graphite; 5 parts of pulp fiber; 3 parts of zinc oxide; 2 parts of sulfur; 1 part of accelerator M; 2 parts of stearic acid; 0.05 parts of potassium hydroxide; 0.05 parts of diffusing agent NF; and 0.5 parts of casein.

[0028] The other preparation steps are the same as those in Example 1-1.

[0029] Examples 1-3 This embodiment provides an example of a natural rubber with high electrical conductivity and a preparation method thereof.

[0030] The raw material ratio of this embodiment is as follows: 100 parts of natural rubber latex; 30 parts of graphite; 5 parts of pulp fiber; 3 parts of zinc oxide; 2 parts of sulfur; 1 part of accelerator M; 2 parts of stearic acid; 0.05 parts of potassium hydroxide; 0.05 parts of diffusing agent NF; and 0.5 parts of casein. The other preparation steps are the same as those in Example 1-1.

[0031] Examples 1-4 This embodiment provides an example of a natural rubber with high electrical conductivity and a preparation method thereof.

[0032] The raw material ratio of this embodiment is as follows: 100 parts of natural rubber latex; 20 parts of graphite; 5 parts of cellulose nanofibers; 3 parts of zinc oxide; 2 parts of sulfur; 1 part of accelerator M; 2 parts of stearic acid; 0.05 parts of potassium hydroxide; 0.05 parts of diffusant NF; and 0.5 parts of casein.

[0033] The other preparation steps are the same as those in Example 1-1.

[0034] Examples 1-5 This embodiment provides an example of a natural rubber with high electrical conductivity and a preparation method thereof.

[0035] The raw material ratio of this embodiment is as follows: 100 parts of natural rubber latex; 20 parts of graphite; 5 parts of wood fiber; 3 parts of zinc oxide; 2 parts of sulfur; 1 part of accelerator M; 2 parts of stearic acid; 0.05 parts of potassium hydroxide; 0.05 parts of diffusing agent NF; and 0.5 parts of casein.

[0036] The other preparation steps are the same as those in Example 1-1.

[0037] Example 2-1 This embodiment provides an example of a natural rubber with high electrical conductivity and a preparation method thereof using a directional freezing technique.

[0038] The difference from Example 1-1 is that in step (5), the mixed solution obtained in step (4) is poured into a mold, first directionally frozen on a one-way cold source at -60°C for 1 h (the one-way cold source contacts any side of the mold, such as the bottom or the side, from top to bottom or from left to right or vice versa, for one-way direction freezing. For example, the mold containing the mixed solution is placed on a cold table, and the cold source of the cold table only contacts the bottom, and is frozen from bottom to top), and then placed in a cold box and low-temperature frozen at -24°C for 24 h.

[0039] The other steps are the same to obtain a natural rubber composite film.

[0040] Example 2-2 This embodiment provides an example of a natural rubber with high electrical conductivity and a preparation method thereof using directional freezing technology. The difference from Example 1-2 is that in step (5), the mixed solution obtained in step (4) is poured into a mold, first directionally frozen on a one-way cold source at -60°C for 1 h (the one-way cold source contacts any side of the mold, such as the bottom or the side, from top to bottom or from left to right or vice versa, for one-way direction freezing. For example, the mold containing the mixed solution is placed on a cold table, and the cold source of the cold table only contacts the bottom, and is frozen from bottom to top), and then placed in a cold box and low-temperature frozen at -24°C for 24 h.

[0041] The other steps are the same to obtain a natural rubber composite film.

[0042] Example 2-3 This embodiment provides an example of a natural rubber with high electrical conductivity and a preparation method thereof using directional freezing technology. The difference from Example 1-3 is that in step (5), the mixed solution obtained in step (4) is poured into a mold, first directionally frozen on a one-way cold source at -60°C for 1 h (the one-way cold source contacts any side of the mold, such as the bottom or the side, from top to bottom or from left to right or vice versa, for one-way direction freezing. For example, the mold containing the mixed solution is placed on a cold table, and the cold source of the cold table only contacts the bottom, and is frozen from bottom to top), and then placed in a cold box and low-temperature frozen at -24°C for 24 h.

[0043] The other steps are the same to obtain a natural rubber composite film.

[0044] Embodiment 2-4 This embodiment provides an example of a natural rubber with high electrical conductivity and a preparation method thereof using directional freezing technology. The difference from Example 1-4 is that in step (5), the mixed solution obtained in step (4) is poured into a mold, first directionally frozen on a one-way cold source at -60°C for 1 h (the one-way cold source contacts any side of the mold, such as the bottom or the side, from top to bottom or from left to right or vice versa, for one-way direction freezing. For example, the mold containing the mixed solution is placed on a cold table, and the cold source of the cold table only contacts the bottom, and is frozen from bottom to top), and then placed in a cold box and low-temperature frozen at -24°C for 24 h.

[0045] The other steps are the same to obtain a natural rubber composite film.

[0046] Embodiment 2-5 This embodiment provides an example of a natural rubber with high electrical conductivity and a preparation method thereof using directional freezing technology. The difference from Example 1-5 is that in step (5), the mixed solution obtained in step (4) is poured into a mold, first directionally frozen on a one-way cold source at -60°C for 1 hour (the one-way cold source contacts any side of the mold, such as the bottom or the side, from top to bottom or from left to right or vice versa, and one-way direction freezing is performed. For example, the mold containing the mixed solution is placed on a cold table, and the cold source of the cold table only contacts the bottom, and is frozen from bottom to top), and then placed in a cold box and low-temperature frozen at -24°C for 24 hours.

[0047] The other steps are the same to obtain a natural rubber composite film.

[0048] Effect analysis The conductivity and mechanical properties of the natural rubber composite film of each embodiment were measured. The thickness of the prepared film was 2 mm. According to GB / T529-2008, the tensile properties of the vulcanized natural rubber were measured using a dumbbell-shaped tensile sample on an electronic tensile testing machine (Guanteng W05), and its conductivity properties were tested. The results are shown in Table 1 below: Table 1 Performance test It can be seen that in the low-temperature freezing solidification molding of Examples 1-1 to 1-3, as the graphite content increases, the conductivity first increases and then decreases, reaching 7.60×10 -4 S / m; at the same time, the tensile properties and elongation at break showed a general downward trend, indicating that too much graphite may have a negative impact on the mechanical properties of rubber.

[0049] Comparing Example 1-2 (5 parts of pulp fiber) and Example 1-4 (5 parts of cellulose nanofiber), the conductivity of cellulose nanofibers was improved from 4.48×10 -4 S / m increased to 5.20×10 -4 S / m, and the tensile properties and elongation at break changed slightly, indicating that the fiber type has a certain influence on the properties of natural rubber.

[0050] Compared with the low-temperature freezing method, the directional freezing technology used in Examples 2-1 to 2-5 significantly improved the tensile properties, elongation at break and conductivity of the natural rubber composite film; the tensile properties of Example 2-1 were improved from 9.04 MPa in Example 1-1 to 16.21 MPa, and the conductivity was improved from 1.8×10 -4 S / m increased to 5.04×10 -3 S / m, indicating that directional freezing technology can effectively improve the comprehensive properties of natural rubber.

[0051] Although the embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and the embodiments. They can be applied to various fields suitable for the present invention. For those familiar with the art, additional modifications can be easily realized.

Claims

1. A method for preparing natural rubber with high electrical conductivity, characterized in that: The raw materials include, by weight: 100 parts of natural latex, 5-40 parts of conductive filler, 0.5-20 parts of cellulose, 1-10 parts of zinc oxide, 0.5-5 parts of sulfur, 0.5-5 parts of accelerator, 1-5 parts of stearic acid, 0.05-1 parts of potassium hydroxide, 0.05-1 parts of diffusing agent NF, and 0.5-20 parts of casein; Step 1, weighing natural rubber latex and preparing it into a solution of a certain concentration, and then removing ammonia from the natural rubber latex by formaldehyde method; Add casein, dispersant NF and potassium hydroxide into water to prepare a solution with a concentration of 10-40%; place zinc oxide, sulfur, accelerator and stearic acid in a ball mill together with casein solution, dispersant NF solution and potassium hydroxide solution for grinding at a speed of 500-3500 rpm and a ball milling time of 2-72 h to obtain a dispersion; add the dispersion into natural rubber latex after ammonia removal treatment, stir for a certain period of time and then ripen at room temperature for 0.5-6 h at a stirring speed of 100-500 rpm to obtain a prepared natural rubber latex solution; Step 2, treating cellulose and preparing a cellulose solution with a concentration of 0.5-5%; Step 3, adding the conductive filler to the cellulose solution obtained in step 2, and dispersing it evenly under high-speed stirring, the stirring speed is 500-3000 rpm, and the stirring time is 0.5-4h to obtain a suspension; Step 4, adding casein into water to prepare a solution with a concentration of 10-40%, and adding the solution to the suspension obtained in step 3, stirring evenly, and then adding the prepared natural latex solution, stirring at room temperature for 0.5-6 h, with a stirring speed of 50-500 rpm, to obtain a mixed solution; Step 5, pour the mixed solution obtained in step 4 into a mold, and freeze it on a low-temperature cold source for 1 to 48 hours, with the freezing temperature being -20°C to -80°C; Step 6, thawing the frozen material obtained in step 5 at room temperature, pre-pressing for 1 to 5 minutes at a pressure of 1 to 5 MPa for multiple times after thawing; drying the pre-pressed material in a 20 to 80° C. forced air drying oven for 3 to 50 hours; placing the material on a vulcanizer for hot pressing vulcanization after drying, the vulcanization pressure is 5 to 15 MPa, the hot pressing temperature is 120 to 145° C., and the hot pressing time is 10 to 30 minutes. After the hot pressing is completed, a conductive natural rubber film is obtained.

2. The preparation method according to claim 1, characterized in that In the low-temperature freezing coagulation and molding process of step 5, a two-step freezing treatment method is adopted, firstly, a cold source is applied in a specific direction of the mixed solution for freeze molding, and then further uniform freezing treatment is performed to freeze-coagulate and fix the natural latex.

3. The preparation method according to claim 2, characterized in that: The freezing treatment method includes one of unidirectional freezing, bidirectional freezing, homogeneous freezing, radial freezing and gradient freezing.

4. The preparation method according to claim 2, characterized in that: The cold source temperature used in the freezing treatment method is -20°C to -80°C.

5. The preparation method according to claim 1, characterized in that: The conductive filler is a carbon-based conductive filler.

6. The preparation method according to claim 1, characterized in that: The cellulose is any one or more of natural cellulose, modified cellulose, nanocellulose, etc., wherein the length of the fiber is ≤5 mm and the diameter is ≤20 μm.

7. The preparation method according to claim 1 or 6, characterized in that: The concentration of the natural rubber latex is 20-65%.

8. The preparation method according to claim 1, characterized in that: The accelerator is one or more of accelerator M, accelerator ZDC, accelerator DM and accelerator DC.

9. A natural rubber with high electrical conductivity obtained by the preparation method according to claim 1.

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