Method for producing a composite matrix via incorporation of carbon nanotubes

Abstract

The effects resulting from the individual components and additives in the first and second mixing are studied for various rubber compounds.SOLUTION: A method of making a composite matrix via incorporation of CNTs is disclosed. The method includes mixing a component comprising CNTs and one or more inorganic surfactants at a first, low shear rate to produce a treated mixture, mixing a component comprising the treated mixture and one or more polymers to produce a polymer-based mixture, and curing the polymer-based mixture to obtain a composite matrix.SELECTED DRAWING: Absent

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of U.S. Provisional Patent Application No. 63 / 065,087, filed August 13, 2020. [Background technology]

[0002] background Carbon nanotubes (CNTs) resemble rolled-up graphene sheets, but are not made from them, and exhibit unique physical and chemical properties that are a direct result of their structure. Consequently, this structure is due to the chiral vectors that nanotubes would form if constructed from actual two-dimensional graphene sheets. The bonding arrangement of graphene, a conjugated plane of angular carbon atoms, limits nanotubes to three possible types, called "zigzag," "armchair," or "chiral," each of which exhibits unique electrical properties: for example, armchair types are highly conductive, while zigzag and chiral types are semiconductive.

[0003] Both single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) can be produced, the latter containing at least one, but often many, concentric SWCNTs. Synthesis conditions vary, resulting in different tube length and diameter distributions, along with the presence of carbonaceous by-products or transition metal catalyst impurities. Regardless of these variations, however, due in part to the inherent chemical properties of CNTs, scaled-up production methods yield bundles of hundreds to thousands of entangled, rather than discrete, CNTs. The inter-tube attractive forces that result in these bundles are believed to be the most significant hurdle to fully exploiting the desirable properties of CNTs when incorporating them into polymers.

[0004] As isolated entities, CNTs have primarily been the focus of academic research, but they also find utility in functional applications when incorporated into polymers to form composites. When incorporated, the as-produced CNTs can impart some of their properties to the encapsulation matrix. This imbuement provides: (1) a strong foundation that binds the tubes together; This is significantly improved when Le Waals forces are interrupted, resulting in uniform dispersion of the CNTs throughout the matrix, and (3) attractive interactions at the CNT / matrix interface are maximized. Two key strategies for achieving these goals have emerged: covalent functionalization of the CNT surface and non-covalent functionalization of the CNT surface.

[0005] Covalent functionalization typically involves severing the conjugated system of CNTs and then adding functional groups (carboxyl, amino, or other). This functionalization allows for stronger attractive interactions in composites (as well as other resident fillers). Covalent functionalization is primarily sp 2 From the hybridized system, sp 3 This requires conversion to a system containing hybridized localizations, the concentration of which increases proportionally with the degree of functionalization. Furthermore, it has been reported that the harsh chemical treatments required to attach these moieties often significantly reduce the aspect ratio of the nanotubes, severely limiting their practical use as fillers in matrices. To date, few, if any, methods have been devised to carry out covalent functionalization methods on a production scale.

[0006] Non-covalent functionalization allows for the modification of CNTs without sacrificing their structure or electron transport capabilities. This provides an alternative approach for exfoliation and incorporation into polymer matrices. The non-covalent approach involves a solvent, preferably a surfactant, that can penetrate the gaps between the bundled CNTs and ensure their solubilized colloidal stability. This mechanism of physical adsorption, rather than chemical reaction, preserves the conjugated structure of the nanotubes, deriving its efficiency from a range of interactions, including but not limited to van der Waals, π-π, and CH-π.

[0007] Several noncovalent functionalization methods have been reported. Favorable interactions, mediated by the appropriate selection of solvents and surfactants, have been shown to promote the exfoliation and stability of debundled nanotubes. The effectiveness of this technique is further enhanced when the process is accompanied by a high-shear-rate mixing step; however, excessively intense shear rates (e.g., high-power jet mixing, sonication, etc.) not only reduce the nanotube aspect ratio but also introduce undesirable impurities along the nanotube sidewalls and end caps, partially negating the benefits of this method. Summary of the Invention [Means for solving the problem]

[0008] Considering the above-mentioned difficulties associated with incorporating CNTs for use in practical applications, this paper describes a new method for fabricating composite matrices, such as rubber compounds, on a production scale based on the efficient non-covalent functionalization of CNTs using various processing techniques. The method described herein is particularly useful for inexpensively fabricating conductive rubber compounds that retain elastomeric properties. [Brief explanation of the drawings]

[0009] BRIEF DESCRIPTION OF THE DRAWINGS [Figure 1] 1 is a flowchart outlining the entire fabrication process. [Figure 2] 1 is a flowchart detailing a process 100 for producing a treated mixture. [Figure 3A]1 is a list of examples of inorganic surfactants, where Me (methyl group) = CH3 and Ph (phenyl group) = C6H5. [Figure 3B] 1 is a list of examples of inorganic surfactants, where Me (methyl group) = CH3 and Ph (phenyl group) = C6H5. [Figure 3C] 1 is a list of examples of inorganic surfactants, where Me (methyl group) = CH3 and Ph (phenyl group) = C6H5. [Figure 3D] 1 is a list of examples of inorganic surfactants, where Me (methyl group) = CH3 and Ph (phenyl group) = C6H5. [Figure 3E] FIG. 1 is a general molecular diagram of a linear polysiloxane. [Figure 3F] FIG. 1 is a general molecular diagram of a branched polysiloxane. [Figure 3G] FIG. 1 is a general molecular diagram of a cyclic polysiloxane. [Figure 4] 3 is a flowchart showing details of step 300 for obtaining a composite matrix. [Figure 5] Photograph of a scanning electron microscope image showing an example of a silicone-based composite matrix containing well-dispersed individual SWCNTs. [Figure 6A] 1 is a photograph showing an example of raw, untreated CNTs. [Figure 6B] 1 is a photograph showing an example of a treated mixture after pressing. [Figure 7A] FIG. 1 is a general molecular diagram of a hydrosilylation reaction precursor having a linear organohydrogenpolysiloxane. [Figure 7B] FIG. 1 is a general molecular illustration of a hydrosilylation reaction precursor based on an inhibitor such as an acetylenic alcohol. [Figure 8] 1 is a table listing the ingredients used to produce Processed Mixture 1. [Figure 9] 1 is a table listing the ingredients used to create Processed Mixture 2. [Figure 10]1 is a table listing ingredients used in a second mix other than the treated mix to produce the base silicone rubber. [Figure 11] 1 is a table listing ingredients used in a second mix other than the treated mix to produce the base fluorosilicone rubber. [Figure 12] 1 is a table listing ingredients used in a second mix other than the treated mix to produce the base EPDM rubber. [Figure 13] 1 is a table listing ingredients used in a second mix other than the treated mix to produce the base nitrile rubber. [Figure 14] 1 is a table listing experimental results regarding the electrical, physical, and rheological properties of various base rubbers and rubber compounds (treated Mixtures 1 or 2). [Figure 15] 1 is a table listing experimental results regarding the electrical, physical, and rheological properties of various base rubbers and rubber compounds (treated Mixtures 1 or 2). [Figure 16] 1 is a table listing experimental results regarding the electrical, physical, and rheological properties of various base rubbers and rubber compounds (treated Mixtures 1 or 2). [Figure 17] 1 is a table listing experimental results regarding the electrical, physical, and rheological properties of various base rubbers and rubber compounds (treated Mixtures 1 or 2). [Figure 18] 1 is a table listing experimental results regarding the electrical, physical, and rheological properties of various base rubbers and rubber compounds (treated Mixtures 1 or 2). [Figure 19] 1 is a table listing experimental results regarding the electrical, physical, and rheological properties of various base rubbers and rubber compounds (treated Mixtures 1 or 2). [Figure 20] 1 is a table listing experimental results regarding the electrical, physical, and rheological properties of various base rubbers and rubber compounds (treated Mixtures 1 or 2). [Figure 21]21 is a chart showing the main characteristics and trends of the characteristics based on the data compiled in FIGS. 14 to 20. [Figure 22] 21 is a chart showing the main characteristics and trends of the characteristics based on the data compiled in FIGS. 14 to 20. [Figure 23] 21 is a chart showing the main characteristics and trends of the characteristics based on the data compiled in FIGS. 14 to 20. [Figure 24] 21 is a chart showing the main characteristics and trends of the characteristics based on the data compiled in FIGS. 14 to 20. [Figure 25] 21 is a chart showing the main characteristics and trends of the characteristics based on the data compiled in FIGS. 14 to 20. [Figure 26] 21 is a chart showing the main characteristics and trends of the characteristics based on the data compiled in FIGS. 14 to 20. DETAILED DESCRIPTION OF THE INVENTION

[0010] Detailed Description This study explores the non-covalent processing of carbon nanotubes (CNTs) to produce a mixture thereof. The resulting processed mixture is then blended with a polymer to produce a composite matrix. This composite matrix can be further processed to create a finished product. The process steps are designed to achieve production-scale manufacturing of polymer-based products, such as high-performance rubber compounds. Initial processing conditions the CNT bundles to exfoliate and debundle them into mostly individual tubes. Other additives can be included in the processed mixture to create a finished product with improved electrical, thermal, mechanical, and other properties. Some examples of this implementation and application are described below. While specific values ​​are cited herein to describe various steps, experiments, and results, it should be understood that these are exemplary values, approximations, and / or values ​​within the tolerances or resolution of the equipment, as can be understood by those skilled in the art.

[0011] Figure 1 is a flowchart outlining the overall fabrication process. First, in step 100, one or more inorganic surfactants are non-covalently functionalized to form C -based nanoparticles, with or without additives. The NT is dispersed in the processed mixture to produce a processed mixture. Details of step 100, how the processed mixture is obtained, will be described later with reference to FIG. 2. The resulting processed mixture can then be used as is or in a pressed form. Pressing includes compaction, pelletization, and other operations to increase bulk density. A pressed form is often required for efficient transportation or shipping. Therefore, in step 200, pressing of the processed mixture can be performed if necessary. Details of the pressing operation and its effects will be described later with reference to FIGS. 6A and 6B. In step 300, a composite matrix is ​​obtained by mixing the processed mixture with one or more polymeric components, with or without additives. Details of step 300, how the composite matrix is ​​produced, will be described later with reference to FIG. 4. After the composite matrix is ​​obtained, a fabrication procedure for the composite matrix is ​​often required depending on the type of finished product. Examples of fabrication procedures include molding, calendaring, and extrusion. In step 400, fabrication of the composite matrix is ​​performed using one or more of the fabrication procedures described above. A finished product having the required properties is thus obtained from the composite matrix in step 500. It should be noted that according to this method of making a composite matrix through the incorporation of CNTs, the various steps in the process shown in the flowchart do not necessarily have to be in the order shown, but can be interchanged, in a different order, or performed in parallel, depending on the convenience of the operation efficiency, application, or any other scenario.

[0012] FIG. 2 is a flowchart detailing step 100 for producing the processed mixture. Steps 104-112 provide the necessary components. Step 104 provides CNTs. Typically, CNTs include single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs), multi-walled carbon nanotubes (MWCNTs), and impurities. To obtain a high-quality finished product, a batch of high-purity SWCNTs is used in this process. An example is the SWCNTs manufactured by Zeon Nano Technology Co., Ltd., which have a carbon purity of 99% or more, an impurity concentration of less than 1%, and a high aspect ratio (average diameter of 3-5 nm and length of 100-600 μm). These include, but are not limited to, SWCNT powder, ZEONANO® SG101.

[0013] Commercially available batches of raw CNTs typically contain bundled CNTs. Therefore, the procedure used to disperse CNTs in solution significantly affects the final suspension properties, which in turn affect the electrical and thermal conductivity of high-performance rubber compounds. As previously mentioned, several technical approaches, including covalent and noncovalent functionalization of CNTs, can be employed to prepare stable, uniform dispersions of CNTs. The noncovalent approach is employed in this process because it leaves the surface structure and electrical conductivity of the CNTs substantially intact. Specifically, through noncovalent functionalization, it is possible to exfoliate and debundle bundled CNTs without substantially sacrificing the intrinsic structure and electron transport capabilities of individual CNTs. This is because the noncovalent approach utilizes solvents or surfactants that can penetrate the gaps between the bundled CNTs through a physical adsorption mechanism rather than a chemical reaction, thereby substantially preserving the intrinsic structure and electrical properties of the individual CNTs.

[0014] In step 108, one or more inorganic surfactants are provided to effect noncovalent functionalization of the CNTs. Specifically, a combination of two or more fluid polymers or a single fluid polymer can be selected to optimally promote attraction to and adsorption along the surface of the CNTs. Each of these fluid polymers can contain a linear, branched, or cyclic polysiloxane backbone and include pendant or terminal substituents. These substituents can be selected to complement each other in their interaction with the CNTs. It has been reported that certain moieties interact strongly with the π-electron-rich surface of the CNTs, allowing less bulky moieties to penetrate the interstices of the CNT bundles, thereby facilitating their exfoliation and debundling. Here, a "moiety" refers to a branch in an organic or inorganic molecule extending from a carbon or siloxane backbone, which can include methyl groups (CH), hydroxyl groups (COH), silanol groups (SiO), and the like. H), aryl groups (such as phenyl and naphthyl groups), or combinations thereof. Figures 3A-3D provide a list of examples of inorganic surfactants where Me (methyl group) = CH3 and Ph (phenyl group) = CH5.

[0015] As described above, this study demonstrates that one or more inorganic surfactants, such as vinyl-terminated polydimethylsiloxane, vinyl-terminated diphenylsiloxane dimethylsiloxane, silanol-terminated polydimethylsiloxane, hydride-terminated polyphenyl-(dimethylhydrosiloxy)siloxane, hydride-terminated polyphenylmethylsiloxane, hydride-terminated polyphenyl-(dimethylsiloxy)siloxane, or combinations thereof, can efficiently penetrate bundled CNTs and adsorb along the surfaces of individual CNTs. These polysiloxane-based fluid polymers exhibit viscosities measured at 25 °C ranging from 0.01 to 10 Pa-s, with the actual viscosity corresponding to the molecular weight of the polymer. In some cases, combining a low-viscosity fluid polymer (e.g., silanol-terminated) with a high-viscosity fluid polymer (e.g., phenylated) is expected to adsorb to individual CNTs more efficiently than using only low-viscosity or only high-viscosity fluid polymers. As will be discussed later, this study demonstrated that optimal results can be achieved by using a silanol-terminated fluid polymer, such as silanol-terminated polydimethylsiloxane, as a single inorganic surfactant, rather than using two or more different inorganic surfactants in combination. Because CNTs remain expensive in today's market, a low weight percent of CNTs in a mixture of CNTs and one or more inorganic surfactants is preferred. In one example, 23 weight percent CNTs and 77 weight percent silanol-terminated polydimethylsiloxane are used.

[0016] Figures 3E, 3F, and 3G show general molecular diagrams of the above surfactants, i.e., linear polysiloxane, branched polysiloxane, and cyclic polysiloxane, respectively, where a is an integer between 0 and 3, and b has a value sufficient to satisfy the above viscosity. In any of the above examples, R can generally include substituted or unsubstituted hydrocarbon groups having 1 to 12 carbon atoms, preferably 1 to 5 carbon atoms, such as alkyl groups such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, hexyl, cyclohexyl, octyl, and dodecyl, aryl groups such as phenyl and tolyl, and halogen-substituted hydrocarbon groups such as 3,3,3-trifluoropropyl. Examples of R' include alkyl, alkenyl, phenyl, hydride, and hydroxyl groups, selected to maximize the desired noncovalent interactions with the CNTs.

[0017] Depending on the properties desired for the finished product, one or more additives can be provided as components of the processed mixture in step 112 to enhance certain properties. Thus, a first mixing in step 116 can be performed to combine the CNTs with one or more inorganic surfactants, with or without one or more additives, referred to herein as first additives. Each of several possible first additives is described below.

[0018] To produce a treated mixture, one or more cure modifiers can be added as a first additive. It has been reported that untreated CNTs can interfere with common (platinum-, peroxide-, or sulfur-based) rubber cure systems. Here, "rubber cure system" refers to the chemical components included in a rubber formulation that enable the formation of a thermoset after a curing procedure, e.g., the application of heat and / or pressure. This interference is not only seen in the finished product, but also in the cure profile. This is particularly evident in certain cured silicone rubbers that undergo hydrosilylation curing reactions, such as platinum-cured silicone rubbers with silicon hydride crosslinkers. Further details regarding optimizing the hydrosilylation cure reaction of silicone rubbers are discussed later in this document, in this case improving CNT encapsulation within the composite matrix.

[0019] The addition of one or more cure modifiers will depend on the type of finished product and the desired properties. Furthermore, they can be added later in the process, for example, just before the curing process, if desired. However, it should be noted that incorporating the required cure modifiers in the processed mixture from the beginning consolidates the preparation steps, which not only improves CNT encapsulation in the composite matrix, but also makes the overall process more efficient.

[0020] Carbon black can be added as the first additive to produce the treated mixture. Carbon black is a material produced by the incomplete combustion of petroleum products and is a form of paracrystalline carbon. It is traditionally used as a reinforcing filler in tires and other rubber products. Examples of high-purity conductive grade carbon black include Tokai Black #5500 (from Tokai, headquartered in Japan) and Denka Black Li-400 (from Denka, headquartered in Japan), which are acetylene-based. Generally, conductive additives such as CNTs or carbon black are used. When black fillers are dispersed in insulating polymers, the electrical percolation threshold is characterized by a rapid drop in electrical resistance by several orders of magnitude. The electrical percolation threshold is related to the formation of an interconnected conductive network of fillers within the host medium. Compared to media filled with carbon black alone, media containing only CNTs, which exhibit much higher aspect ratios, can achieve the electrical percolation threshold at a much lower filler percentage. However, incorporating both CNTs and carbon black into a host medium has been reported to produce synergistic effects resulting from their respective contributions to the formation of an interconnected conductive network. As shown in the examples below, carbon black added to processed mixtures acts synergistically with CNTs to bridge electron transport pathways and strengthen the conductive network in the composite matrix, more so than CNTs alone.

[0021] A partitioning agent can be added as a first additive to generate the processed mixture. Examples of nanoscale or microscale partitioning agents include glass beads, glass bubbles, and conductive metal powders. Examples of such partitioning agents include 3M™ Glass Bubbles iM30K and Glass Bubbles iM16K (available from 3M, headquartered in the United States). The addition of these ingredients is expected to facilitate debundling of CNTs during the subsequent first mixing stage 116 due to a ball-bearing grinding effect. In addition, the incorporation of glass beads (GBs) into a silicone-MWCNT combination has been reported to significantly improve the dispersion of MWCNTs in silicone. Specifically, the electrical conductivity of a silicone / MWCNT / GB composite was approximately twice that of a composite without GBs due to improved uniformity of MWCNT dispersion in silicone. Furthermore, the presence of GBs is expected to improve the mechanical properties of the composite, such as tensile strength and elongation at break, in addition to electrical conductivity. As shown in the examples below, a distribution agent, e.g., glass bubbles, added to the processed mixture improves the uniformity of the dispersion of CNTs, e.g., SWCNTs, thereby improving the electrical conductivity and mechanical properties of the composite matrix.

[0022] A thickening agent or blowing agent can be added as a first additive to produce the treated mixture. Examples of thickening agents or blowing agents include foam formers, expandable cell composites, and other void modifiers, which function to increase void space in the composite matrix, strengthen the interconnecting conductive network, and improve electrical conductivity. Examples of thermally expandable thermoplastic microspheres are available from Expancel® Microsphere Products (headquartered in the Netherlands). Put a propellant (commonly known as Nouryon brand) into the capsules. Considering the overall process flow, expanding spheres are added to produce a treated mixture in a first mix 116, then the treated mixture is mixed with one or more polymers in a second mix 316, and then during the process, upon heating, the spheres first expand, and then the curing 320 of the composite matrix permanently fixes the expanded cells. Foaming agents, such as those based on methyl-phenyl or diphenyl-phenyl groups, have been shown to provide fine foaming and cellular structure in cured fabricated products. These agents can be added as the initial additive to produce processed mixtures in a similar manner to composites containing expandable cells. With silicones in particular, foam-forming agents can be used to create voids upon heating and harden during the process. These agents include combinations of water, silicon hydride crosslinkers, and phenylsilicones. Phenylsilicone fluids are selected from those consisting of linear polysiloxane chains with methyl-phenyl or diphenyl pendant groups substituted with dimethyl groups along the polysiloxane backbone, and viscosities measured at 23°C ranging from 0.01 to 10 Pa-s. When added to produce processed mixtures and then incorporated into the polymer matrix, these agents effectively foam the composite matrix through the creation of numerous nucleation sites that result in the creation of fine foam cells that grow to maintain a surface skin upon cure.

[0023] After all necessary ingredients are prepared in steps 104-112 above, the ingredients are placed in a first mixer and mixed in the first mixing step 116 of Figure 2. The first mixer can be a cone mixer or pin mixer with a simple mixing mechanism. Experiments have shown that the use of such a simple, low-cost mixer with shear rates in the range of 100-100,000 1 / s for a period ranging from 5 minutes to 1 hour results in gentle debundling of the CNTs, resulting in a high-quality dispersion with minimal damage to their conjugated structure. The processed mixture is then obtained in step 120.

[0024] Conventional mixing methods for noncovalent functionalization typically involve the use of jet mixers, ultrasonicators, or other costly, high-power machinery with very high shear rates. For example, typical shear rates generated by jet mixers exceed 100,000 1 / s, which often damages CNTs. Ultrasonication with ultrasonicators also shortens CNTs, thereby reducing their aspect ratio and significantly reducing their usefulness as conductive fillers. Furthermore, ultrasonication naturally generates heat, necessitating cooling during the procedure. In contrast, as described above with reference to the first mixing step 116, a simple, low-cost mixer with low shear rates is used in the present process to mix the CNTs with other ingredients, minimizing breakage and resulting in a processed mixture containing substantially debundled CNTs with high-quality dispersion properties.

[0025] 4 is a flowchart detailing step 300, where processing steps are performed to create a composite matrix. In steps 304 and 308, the necessary components are provided. In step 304, one or more polymers are provided. In this process, a combination of two or more polymers can be used to form the base polymer matrix to modify certain properties, for example, to increase strength after curing. Examples of polymers that can be used in the present process include polysiloxanes (with methyl, trifluoropropyl, or phenyl substituents), ethylene-propylene copolymers, ethylene-propylene-diene terpolymers, acrylonitrile-butadiene copolymers, styrene-butadiene copolymers, isoprene polymers, isobutylene-isoprene copolymers, chloroprene polymers, butadiene polymers, chlorinated polyethylene polymers, epichlorohydrin polymers, ethylene-acrylic copolymers, polyacrylate copolymers, ethylene-vinyl acetate copolymers, polypropylene oxide copolymers, fluorocarbon elastomeric copolymers, tetrafluoroethylene copolymers, perfluoro-elastomeric copolymers, polyether-urethane polymers, polyester-urethane polymers, other commercially available polymers or copolymers, and any combination thereof.

[0026] In step 308, the processed mixture is provided in the as-produced form obtained in step 100 or in the pressed form obtained in step 200. In step 312, the finished product is Depending on the desired properties, other ingredients can be added to enhance specific properties. Thus, the second mixing in step 316 can be performed to mix one or more polymers with the processed mixture obtained according to the previous step, as shown in FIG. 2, with or without one or more additives, referred to herein as second additives. Examples of such second additives include fillers, plasticizers, stabilizers, cure initiators, cure modifiers, cure accelerators, catalysts, curing agents, and any combination thereof. Examples of fillers include silica, fumed silica, nanosilica, functionalized or non-functionalized silicone resins, natural and synthetic fibers, polysaccharides, cork, graphite and carbon black, graphene, clay, boron nitride, finely divided metals and metal oxides, and any combination thereof.

[0027] After preparing all the necessary ingredients in steps 304-312 above, the ingredients are placed in and mixed in a second mixer in a second mixing step 316 of Figure 4. The second mixer can be a conventional rubber processing mill or mixer with a low shear rate of less than 100 1 / sec to obtain a polymeric mixture, i.e., an as-processed, raw composite matrix before curing, with substantially debundled CNTs and other ingredients.

[0028] In step 320, the polymeric mixture produced by the mixing is subjected to a curing process, during which it irreversibly hardens to produce a thermoset. Typically, curing is induced by heat or suitable irradiation and can be accelerated by high pressure or mixing with a catalyst. A catalyst can be added in the second mixing step 316. A transition metal-based catalyst synthesized from chloroplatinic acid and chloroplatinic acid can be used for this curing. The curing is based on a chemical reaction that causes extensive cross-linking between polymer chains to produce a substantially injectable and insoluble polymer network. Thus, a composite matrix with the intended properties is stabilized and obtained in step 324. Figure 5 shows a scanning electron microscope image of an example of a silicone-based composite matrix containing well-dispersed individual SWCNTs. The composite matrix can then be subjected to fabrication procedures such as molding, calendering, and extrusion, as required, in step 400 of Figure 1, ultimately resulting in a finished product in step 500 of Figure 1.

[0029] Referring back to the pressing procedure of step 200 in FIG. 1 , pressing can include compression, pelletizing, and other bulk density-increasing operations. An example of a pelletizing procedure involves the use of a pelletizer to cylindrically form the processed mixture into pellets. A binder, such as a cyclic polysiloxane, can be added to the pelletizer. The resulting pellets are heated to remove the binder at a temperature below the activation temperature of the other additives in the processed mixture, i.e., the first additive. After cooling, the pellets can be packaged for shipping. However, it should be noted that surfactants often function sufficiently to function as binders. Therefore, the addition of a specific binder, such as a cyclic polysiloxane, may not be necessary, and therefore, in this case, the subsequent heating and cooling processes are not necessary. Instead of pelletizing, light pressing or tamping can be performed to increase bulk density. Generally, shipping and handling of commercially available raw CNTs, e.g., SWCNT powder, is inefficient due to their extremely low bulk density, resulting in lightweight raw CNTs occupying large cargo space.

[0030] FIG. 6A is a photograph showing an example of raw, unprocessed CNTs, with a mass of 7.122 g and an occupied volume of approximately 270 cm. 3 Therefore, the bulk density is about 26 kg / m 3 FIG. 6B is a photograph showing an example of the treated CNTs, i.e., the treated mixture after pressing in step 200, which in this case has a mass of 19.339 g and occupies a volume of approximately 91 cm. 3 Therefore, the bulk density is about 213 kg / m 3 The pressing operation herein involves tamping or pressing using manual or mechanical pressure without the use of an additional binder to achieve a compression ratio of greater than 8 times, e.g., about 8.2.

[0031] Referring back to step 112 of Figure 2, as previously described, one or more cure modifiers can be added when preparing the treated mixture. In this study, the hydrosilylation cure reaction in silicone rubber is optimized by maintaining a stoichiometric balance of Si-H to vinyl. An imbalanced hydrosilylation cure system can result in either an under-cure condition, resulting in poor elastomeric properties, or an over-cure condition, resulting in undesirable adhesion to mold surfaces and other substrates. As noted, hydride-functional polysiloxanes form non-covalent interactions with CNTs; in addition, they are also hydrosilylation reaction precursors (described as crosslinkers). As a result, a competition occurs between the cure reaction and the affinity of CNTs for the crosslinker. This competition can disrupt the stoichiometric balance over time due to different reaction rates.

[0032] An additional problem arises when processing composite matrices containing CNTs. As mentioned, the composite matrix is ​​optimized when carbon nanotubes are exfoliated from the CNT bundles into separate individual tubes. These individual tubes must also be fully encapsulated within the composite matrix. Any exposed CNTs, or uncured composite matrices containing CNTs, can result in the undesirable surface effect of the exposed tubes being removed by abrasion or scraping, leaving black marks on the surface they come into contact with. This is sometimes referred to as sloughing, because the CNTs produce a black, oily residue along the surface of the composite matrix.

[0033] To remedy the adverse effects of CNTs on the composite matrix, the present study involves using a cure modifier consisting of a hydrosilylation reaction precursor containing a hydrosilylation crosslinker and a reaction inhibitor, which is added to the mixture processed in the first mixing step 116. The addition of the crosslinker compensates for absorption through the CNTs and balances the Si-H to vinyl stoichiometry in the hydrosilylation reaction. The addition of the inhibitor prevents scorch (defined as premature cure). The composite matrix can be cured at higher temperatures without causing undesirable problems such as catalysis (crack formation). These higher process temperatures allow for more extensive and complete cross-linking, resulting in more complete encapsulation of individual CNTs by the polymer link. The surface of the composite matrix is ​​clean and contains fully encapsulated CNTs. Hydrosilylation inhibitors have been reported to inhibit the activation of transition metal catalysts synthesized from platinum chloride and chloroplatinic acid. These platinum divinyltetramethyldisiloxane complexes, known as Karstedt catalyst complexes, are typically: platinum(0)-1,3-divinyl,1,1,3,3-tetramethyldisiloxane, platinum(0)-2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane complex, bis(acetylacetonato)platinum, (m-cyclopentadienyl)trialkylplatinum complexes, platinum triazenide complexes, platinum, iron, palladium, and rhodium complexes, or any combination thereof.

[0034] The following formulations illustrate how the addition of a crosslinker and inhibitor as a cure modifier can be performed, specifically using a hydrosilylation reaction precursor consisting of a hydrosilylation crosslinker and a reaction inhibitor. Note that phr in the tables below is defined as parts per hundred rubber; values ​​in the formulations can be converted to percent by dividing the amount of the component by the total amount and multiplying by 100.

[0035] Table 1 lists the phr values ​​used in Formulation Example 1, which is a less than optimal formulation that results in a composite matrix characterized by high adhesion to molds and other substrates and poor CNT encapsulation (degradation).

[0036] [Table 1]

[0037] Table 2 lists the phr values ​​used for Formulation Example 2, a more optimal formulation, which results in a composite matrix characterized by low adhesion to molds and other substrates and good CNT encapsulation.

[0038] [Table 2]

[0039] In Tables 1 and 2 above, the "First Mixing" column refers to step 116 of mixing CNTs and inorganic surfactants with or without one or more first additives to produce a treated mixture, the "Second Mixing" column refers to step 316 of mixing the treated mixture with a polymer with or without one or more second additives to create a composite matrix, and the "Combination of First Mixing and Second Mixing" column refers to step 316 of mixing the treated mixture with a polymer with or without one or more second additives to create a composite matrix. The combined total of all ingredients in both is listed.

[0040] In the "First Mix and Second Mix Combination" column of Table 2, the crosslinker level was reduced from 3.55 phr to 0.73 phr and added only in the first mix, while the inhibitor level was increased from 0.28 phr to 0.56 phr, with half of that amount added to each of the first and second mixes. This experiment demonstrates that cure modifiers added in this order, first in the first mix as part of the processed mix, better maintain the Si-H to vinyl stoichiometric balance, allowing for improved CNT encapsulation in the resulting composite matrix.

[0041] To further illustrate the effect of the cure modifier on the hydrosilylation reaction precursor, the following table shows how the addition of a cure modifier in the first mix affects the cure start time (TS2) and cure completion time (TC90). These cure rates are directly related to cure efficiency. Generally, the inclusion of CNTs in the mix tends to decrease cure efficiency. One solution to this problem is to add the cure modifier in the second mix, as in the example formulation in Table 1 above, but this often results in a composite matrix with high adhesion to molds and other substrates and poor CNT encapsulation (degradation). A preferred method is to add the cure modifier in the first mix, as in Table 2. Table 3 below shows that adding a cure modifier in this way improves cure efficiency, with cure rates approaching those for the "no treated CNT mix" case, e.g., TS2 = 30 seconds and TC90 = 66 seconds (177°C). Specifically, the levels of crosslinker and inhibitor of the cure modifier in the first mix can be adjusted to restore cure efficiency and provide high throughput, thereby achieving an economically produced composite matrix.

[0042] [Table 3]

[0043] The cure modifier composition can include a hydrosilylation reaction precursor having a linear organohydrogenpolysiloxane, a general molecular illustration of which is shown in FIG. 7A, where y is an integer from 1 to 98, z is an integer from 2 to 50, and y+z is 9 to 100; R2 is independently an optionally substituted monovalent hydrocarbon group containing 1 to 10 carbon atoms; and R3 is R2 or a hydrogen atom.

[0044] FIG. 7B is a general molecular illustration of another hydrosilylation reaction precursor based on an inhibitor such as an acetylenic alcohol, in which the same or different substituents R1 and R2 independently represent a linear or branched monovalent alkyl, cycloalkyl, (cycloalkyl)alkyl, aromatic, or arylalkyl group, which can be joined in pairs to form a 5-, 6-, 7-, or 8-membered aliphatic ring optionally substituted with one or more substituents.

[0045] In this study, various experiments were carried out to produce rubber compounds using the above-mentioned method based on the incorporation of CNTs, and their properties and characteristics were analyzed to understand the effects due to CNTs and various additives. The results and special technical features are described below with reference to Figures 8 to 26.

[0046] As previously described with reference to FIG. 2, two processed mixtures, Processed Mixture 1 and Processed Mixture 2, are produced based on the first mixture. FIGS. 8 and 9 are tables listing the components used to produce Processed Mixture 1 and Processed Mixture 2, respectively. The weight percent (wt%) of each component in the processed mixture is shown in the right-most column of each table. The CNTs used herein are carbon nanotubes with a carbon purity of 99%. % or more of SWCNT powder is used, and the terms CNT and SWCNT are used interchangeably in the tables and charts of Figures 8-26. Carbon black and glass bubbles are used as the first additive to produce both Processed Mixtures 1 and 2. Processed Mixture 1 contains 30 wt% of one inorganic surfactant. Processed Mixture 2 contains 10 wt% each of three inorganic surfactants.

[0047] The processed mixture produced based on the first mix is ​​then mixed with one or more polymers and a second additive based on the second mix to produce a polymer-based mixture, which is cured to produce a composite matrix, i.e., a rubber compound in this experiment, as described above with reference to FIG. 4. Fabrication procedures, such as molding, calendaring, and extrusion, are then performed as needed to process the composite matrix and produce the final product, in this case, a rubber product. FIGS. 10-13 are tables listing the ingredients used in the second mix, other than the processed mixture, to produce base silicone rubber, base fluorosilicone rubber, base EPDM rubber, and base nitrile rubber, respectively. That is, each of FIGS. 10-13 is a table showing a base rubber formulation, including one or more polymers and a second additive, that is mixed with the processed mixture in the second mix to produce a rubber compound. The weight percent of each ingredient in the base rubber is shown in the rightmost column of each table.

[0048] 14-20 are tables listing experimental results for the electrical, physical, and rheological properties of various base rubbers and rubber compounds, including treated Mixtures 1 or 2. The rheological properties are presented in terms of ML (minimum torque), MH (maximum torque), TS2 (time to start cure), and TC90 (time to complete cure).

[0049] Figure 14 includes details of the formulations of the base silicone rubber and silicone rubber compounds obtained by mixing the base silicone rubber formulation with treated mixture 1 at effective SWCNT weight percentages of 1 wt%, 2 wt%, and 3 wt%, respectively. Figure 15 includes details of the formulations of the base silicone rubber and silicone rubber compounds obtained by mixing the base silicone rubber formulation with treated mixture 2 at effective SWCNT weight percentages of 1 wt%, 2 wt%, and 3 wt%, respectively. Here, effective weight percentages are the weight percentages of the component in the total weight of all components of each rubber compound.

[0050] FIG. 16 shows the results of blending raw SWCNTs with the base silicone rubber formulation without any treatment (i.e., without first blending with one or more inorganic surfactants) at base silicone rubber and effective SWCNT weight percentages of 1 wt %, 2 wt %, and 3 wt %, respectively. The details of the formulation of the silicone rubber compound obtained are included.

[0051] FIG. 17 contains details of the formulation of silicone rubber compounds obtained by mixing the base silicone rubber formulation with the raw SWCNTs and raw carbon black without treatment (i.e., without first mixing with one or more inorganic surfactants), with base silicone rubber and 1 wt % raw SWCNTs and 3.1 wt % raw carbon black, and 2 wt % raw SWCNTs and 7.1 wt % raw carbon black (CB), respectively.

[0052] FIG. 18 includes details of the base fluorosilicone rubber and the formulation of the fluorosilicone rubber compound obtained by mixing the base fluorosilicone rubber formulation with treated mixture 1 at effective SWCNT weight percentages of 1 wt%, 2 wt%, and 3 wt%, respectively.

[0053] FIG. 19 includes details of the formulation of the base EPDM rubber and the EPDM rubber compounds obtained by mixing the base EPDM rubber formulation with treated Mix 1 at effective SWCNT weight percentages of 0.9 wt%, 1.7 wt%, and 3 wt%, respectively.

[0054] FIG. 20 includes details of the base nitrile rubber and the nitrile rubber compound formulations obtained by mixing the base nitrile rubber formulation with treated mixture 1 at effective SWCNT weight percentages of 0.9 wt%, 2 wt%, and 3 wt%, respectively.

[0055] Figures 21-26 are charts showing key features and trends in properties based on the data summarized in Figures 14-20. Note that the weight percent of CNTs in these charts corresponds to the effective weight percent of SWCNTs in the tables of Figures 14-20.

[0056] Figure 21 is a chart showing the volume resistivity as a function of CNT weight percentage for silicone rubber compounds containing processed mixture 1, processed mixture 2, pristine CNT, and pristine CNT plus pristine carbon black (CB), as well as the base silicone rubber based on the base silicone rubber formulation. When the weight percentages of CNT and CB are equal, both processed mixture 1 and processed mixture 2 exhibit lower volume resistivity than either the silicone rubber compound containing pristine CNT or the silicone rubber compound containing pristine CNT plus pristine CB. It also shows that the electrical percolation threshold for both processed mixture 1 and processed mixture 2 is met at a lower weight percentage of CNT than either the silicone rubber compound containing pristine CNT or the silicone rubber compound containing pristine CNT plus pristine CB. It also shows that the carbon black added to the processed mixture functions synergistically with the CNT to bridge the electron transport pathway and strengthen the conductive network in the composite matrix compared to the case of CNT alone. Furthermore, a distribution agent, such as glass bubbles, added to the processed mixture can improve the uniformity of the CNT dispersion, thereby enhancing the electrical conductivity in the rubber compound.

[0057] FIG. 22 is a chart showing the durometer as a function of CNT weight percent in the base silicone rubber based on Treated Mix 1, Treated Mix 2, pristine CNT, and pristine CNT + pristine carbon black (CB) silicone rubber compounds, and the base silicone rubber formulation. Typical rubber applications fall within a durometer range of 40-70 Shore A. Referring to FIGS. 21 and 22, both the silicone rubber compounds containing Treated Mix 1 and Treated Mix 2 exhibit durometers within this desirable range, while the silicone rubber compounds containing either pristine CNT or pristine CNT + pristine CB exhibit durometers within this desirable range. In comparison, the electrical percolation threshold is met at a low wt % of CNTs.

[0058] Figure 23 is a chart showing tensile strength as a function of CNT weight percentage for silicone rubber compounds containing processed mixture 1, processed mixture 2, pristine CNTs, and pristine CNTs plus pristine carbon black (CB), as well as the base silicone rubber formulation. Referring to Figures 21 and 23, both processed mixture 1 and processed mixture 2 meet the electrical percolation threshold at lower weight percentages of CNTs than either the pristine CNT silicone rubber compound or the pristine CNTs plus pristine CB silicone rubber compound, while still maintaining appreciable tensile strength. Typically, many rubber applications requiring electrically conductive materials do not prioritize tensile strength as a required feature, and a tensile strength of 400 psi is often considered sufficient for such applications.

[0059] FIG. 24 is a chart showing the elongation at break as a function of CNT weight percentage for silicone rubber compounds containing processed mixture 1, processed mixture 2, raw CNTs, and raw CNTs plus raw carbon black (CB), as well as the base silicone rubber based on the base silicone rubber formulation. The silicone rubber compound containing processed mixture 1 exhibits improved elastomeric properties compared to the other cases. For example, referring to FIGS. 21 and 24, at 2 wt% CNTs, the silicone rubber compound containing processed mixture 1 exhibits the lowest volume resistivity (highest conductivity) and the highest elongation at break. Furthermore, in the range of 0 wt% CNT to 3 wt% CNT, including 1 wt% and 2 wt% CNTs, the silicone rubber compound containing processed mixture 1 exhibits the highest elongation at break. It can also be seen that adding a distribution agent, such as glass bubbles, to the processed mixture improves the uniformity of CNT dispersion, thereby improving the mechanical properties of the rubber compound.

[0060] Figure 25 is a chart showing tear strength as a function of CNT weight percent for treated Mix 1, treated Mix 2, pristine CNT, and pristine CNT + pristine carbon black (CB) silicone rubber compounds, and a base silicone rubber based on the base silicone rubber formulation. The silicone rubber compound containing treated Mix 1 exhibits tear strength comparable to the base silicone rubber at 1 and 2 weight percent CNT. Referring to Figures 1 and 25, this tear strength is not significantly compromised at the CNT weight percent required to meet the electrical percolation threshold.

[0061] FIG. 26 is a chart showing volume resistivity as a function of CNT weight percent for a silicone rubber compound with treated Mix 1, a fluorosilicone rubber compound with treated Mix 1, an EPDM rubber compound with treated Mix 1, and a nitrile rubber compound with treated Mix 1. Using the methods described herein and treated Mix 1, the silicone rubber compound, the fluorosilicone rubber compound, and the nitrile rubber compound all pass the electrical percolation threshold at 0-2 wt % CNT, while the EPDM rubber compound passes the electrical percolation threshold at 2-3 wt % CNT.

[0062] While this document contains many specific descriptions, these should not be construed as limitations on the scope of the invention or what may be claimed, but rather as descriptions of features inherent in particular embodiments of the invention. Certain features that are described herein in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Furthermore, features may be described above as working in particular combinations and may not initially be claimed as such. Although it may be the case that one or more features from a claimed combination may in some cases be exercised from the combination, the claimed combination may be directed to a subcombination or a variation of the subcombination.

Claims

1. 1. A method of making a composite matrix, comprising: a first mixture comprising carbon nanotubes (CNTs) and one or more inorganic surfactants to form a treated mixture, the first mixture being configured to have a low shear rate, the one or more inorganic surfactants comprising moieties characterized by an ability to interact with the surfaces of the CNTs and penetrate interstices of the bundled CNTs, thereby facilitating exfoliation and debundling of the bundled CNTs; second, mixing the treated mixture with a component comprising one or more polymers to form a polymer-based mixture; curing the polymer-based mixture to obtain the composite matrix; A method comprising:

2. The method of claim 1, wherein the low shear rate is in the range of 100 to 100,000 1 / sec.

3. 10. The method of claim 1, wherein the first mixing uses a cone mixer or a pin mixer.

4. 10. The method of claim 1, wherein the second mixing uses a rubber mixing mill or mixer.

5. The method of claim 1 , wherein the moieties comprise methyl groups, hydroxyl groups, silanol groups, aryl groups, or combinations thereof.

6. 10. The method of claim 1, wherein the one or more inorganic surfactants comprise vinyl-terminated polydimethylsiloxane, vinyl-terminated diphenylsiloxane dimethylsiloxane, silanol-terminated polydimethylsiloxane, hydride-terminated polyphenyl-(dimethylhydrosiloxy)siloxane, hydride-terminated polyphenylmethylsiloxane, hydride-terminated polyphenyl-(dimethylsiloxy)siloxane, or a combination thereof.

7. 7. The method of claim 6, wherein the one or more inorganic surfactants is an inorganic surfactant comprising a silanol-terminated polydimethylsiloxane.

8. 10. The method of claim 1, wherein the first mixing combines components including the CNTs, the one or more inorganic surfactants, and one or more first additives to produce the treated mixture.

9. The method of claim 8 , wherein the one or more first additives comprise one or more cure modifiers to improve encapsulation of the CNTs in the composite matrix.

10. 10. The method of claim 9, wherein the one or more cure modifiers comprise a hydrosilylation-reactive precursor, a peroxide-reactive precursor, a sulfur-reactive precursor, or a combination thereof.

11. 10. The method of claim 9, wherein the one or more cure modifiers comprise a hydrosilylation reaction precursor consisting of a hydrosilylation crosslinker and a reaction inhibitor.

12. 10. The method of claim 8, wherein the one or more first additives comprise carbon black to achieve a synergistic effect with the CNTs to form an interconnecting conductive network to improve electrical conductivity.

13. 10. The method of claim 8, wherein the one or more first additives comprise a partitioning agent to promote uniformity of dispersion of the CNTs in the composite matrix to improve electrical conductivity.

14. The method of claim 13 , wherein the dispensing agent comprises glass beads, glass bubbles, or conductive metal powder.

15. 10. The method of claim 8, wherein the one or more first additives comprise a thickening agent or a foaming agent to alter voids in the composite matrix to strengthen an interconnecting conductive network and improve electrical conductivity.

16. The method of claim 15, wherein the foaming agent comprises a foam former or a composite material containing expandable cells to increase void space.

17. 10. The method of claim 1, wherein the second mixing comprises mixing components including the treated mixture, the one or more polymers, and one or more second additives to produce the polymer-based mixture.

18. 20. The method of claim 17, wherein the one or more secondary additives comprise a filler, a plasticizer, a stabilizer, a cure initiator, a cure modifier, a cure accelerator, a catalyst, a curing agent, or a combination thereof.

19. 20. The method of claim 18, wherein the filler comprises silica, fumed silica, nanosilica, silicone resins, natural and synthetic fibers, polysaccharides, cork, graphite, carbon black, graphene, clay, boron nitride, finely divided metals and metal oxides, or combinations thereof.

20. 20. The method of claim 17, wherein the one or more second additives comprise a portion of a cure modifier included in the first mix.

21. 21. The method of claim 20, wherein the portion of the cure modifier is a reaction inhibitor of a hydrosilylation reaction precursor consisting of a hydrosilylation crosslinker and a reaction inhibitor, and the hydrosilylation crosslinker and the reaction inhibitor are used in the first mixing.

22. 10. The method of claim 1, wherein the one or more polymers comprise polysiloxanes having methyl, trifluoropropyl, or phenyl substituents, ethylene-propylene copolymers, ethylene-propylene-diene terpolymers or polymers, acrylonitrile-butadiene copolymers, styrene-butadiene copolymers, isoprene polymers, isobutylene-isoprene copolymers, chloroprene polymers, butadiene polymers, chlorinated polyethylene polymers, epichlorohydrin polymers, ethylene-acrylic copolymers, polyacrylate copolymers, ethylene-vinyl acetate copolymers, polypropylene oxide copolymers, fluorocarbon elastomeric copolymers, tetrafluoroethylene copolymers, perfluoroelastomeric copolymers, polyether-urethane polymers, polyester-urethane polymers, or combinations thereof.

23. 10. The method of claim 1, wherein the CNTs comprise at least 99% single-walled carbon nanotubes (SWCNTs) in powder form.

24. The method of claim 1 further comprising pressing the treated mixture after the first mixing.

25. 25. The method of claim 24, wherein the pressing comprises tamping to increase the bulk density of the treated mixture by more than 8 times compared to the CNTs.

26. 1. The rubber compound made using a method comprising: mixing a first component comprising carbon nanotubes (CNTs) and one or more inorganic surfactants to form a treated mixture, the first mixture configured to have a low shear rate, the one or more inorganic surfactants comprising moieties characterized by an ability to interact with surfaces of the CNTs and penetrate interstices of the bundled CNTs, thereby facilitating exfoliation and debundling of the bundled CNTs; mixing a second component comprising the treated mixture and one or more polymers to form a polymer-based mixture; and curing the polymer-based mixture to obtain a rubber compound; the volume resistivity of the rubber compound is lower than the volume resistivity of a rubber compound made using pristine CNTs without the first mixing; The rubber compound has an electrical percolation threshold that is met at a weight percentage of CNTs that is lower than the weight percentage of CNTs at which the electrical percolation threshold of a rubber compound made using pristine CNTs without the first blending is met.

27. the components in the first mixture include the CNTs, an inorganic surfactant, carbon black, and glass bubbles to produce the treated mixture; 27. The rubber compound of claim 26, wherein the electrical percolation threshold is met at 0-2 wt. % CNTs when the one or more polymers in the second mixture comprise a silicone rubber base, a fluorosilicone rubber base, or a butadiene-acrylonitrile copolymer, and at 2-3 wt. % CNTs when the one or more polymers in the second mixture comprise an ethylene propylene diene polymer.

28. the components in the first mixture include the CNTs, an inorganic surfactant, carbon black, and glass bubbles to produce the treated mixture; the one or more polymers in the second blend include a silicone rubber base; 27. The rubber compound of claim 26, wherein in the range of 0 wt% to 3 wt% CNT, the elongation at break of the rubber compound is higher than the elongation at break of a rubber compound made using raw CNT without the first compound and using a silicone rubber base in the second compound.

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