A sizing agent for carbon fibers and a method for preparing the same

Abstract

The present application relates to the technical field of carbon fiber surface treatment, and particularly relates to a sizing agent for carbon fiber and a preparation method thereof, the sizing agent for carbon fiber comprises 20-40 parts by weight of a main material and deionized water to make up to 100 parts by weight; the main material comprises the following components in percentage by mass: 70-78% of an epoxy resin composition, 10-14% of a modified polyester emulsifier, 10-14% of a functional interface modifier, 1-3% of an auxiliary additive group, and the sum of the percentage by mass of the above components is 100%. The present application can give the sizing agent for carbon fiber good bunching of carbon fiber tows, easy uniform yarn spreading in the process of prepreg preparation, and excellent interface bonding performance and storage stability.

Description

Technical Field

[0001] This invention belongs to the field of carbon fiber surface treatment technology, and particularly relates to a sizing agent for carbon fiber and its preparation method. Background Technology

[0002] Carbon fiber possesses excellent properties such as high specific strength, high specific modulus, high temperature resistance, and corrosion resistance, making it an indispensable key reinforcing material in aerospace, new energy vehicles, wind power generation, and high-end sporting goods. In the production and application of carbon fiber, sizing agents play a crucial role—they not only protect the carbon fiber bundles from abrasion, prevent fuzzing and breakage, but also determine the quality of the interfacial bonding between the carbon fiber and the resin matrix, directly affecting the mechanical properties of the final composite material.

[0003] With the increasing demand for lightweighting in fields such as aerospace-grade prepregs and high-end sporting goods, and the expanding application of ultra-low areal density prepregs (hereinafter referred to as "high-end prepregs") such as 50g / m², 75g / m², and even 30g / m², higher requirements are being placed on the processing performance of carbon fibers. Sizing agents are a key factor in meeting these requirements. In the field of high-end prepregs, sizing agents need to simultaneously consider the following mutually restrictive needs: Firstly, there is the challenge of balancing the bundle's cohesiveness and its spreading properties. In the carbon fiber bundle state, the sizing agent needs to impart sufficient cohesiveness to ensure the bundle maintains its shape and is less prone to dispersion and fuzzing during processes such as unwinding, conveying, and rolling, thereby reducing fuzz and breakage rates. However, during prepreg preparation, the carbon fiber bundle needs to be fully spread (spread) to form a uniform thin layer, ensuring sufficient resin impregnation and consistent prepreg areal density. If the cohesiveness is too strong, the bundle is difficult to spread, and the resin cannot penetrate the fiber interior, resulting in uneven fiber distribution and insufficient resin impregnation in the prepreg. If the cohesiveness is too weak, the bundle is prone to dispersion and breakage during processing, affecting production efficiency and prepreg quality. This contradiction between cohesiveness and spreading properties has long been a core technical challenge in the field of carbon fiber sizing agents.

[0004] Secondly, there is the processing adaptability of ultra-thin prepregs. With the increasing demand for lightweighting in fields such as aerospace-grade prepregs and high-end sporting goods, the need for ultra-low areal density prepregs of 50g / m², 75g / m², and even 30g / m² is becoming increasingly urgent. These ultra-thin prepregs require carbon fiber tows to form a uniform, seamless fiber layer after unwinding. If the unwinding is uneven, overlaps or gaps will occur between the fiber bundles, severely affecting the precise control of the prepreg's areal density and the stability of the composite material's mechanical properties. However, existing sizing agents often fail to achieve ideal unwinding effects when applied to 24K or even 48K large-tow carbon fibers. The tows either become too tightly clustered and difficult to unwind, posing a risk of hardening during storage, or develop gaps and unevenness after unwinding.

[0005] Thirdly, the synergy between interfacial and processing properties. High-end prepregs place higher demands on the interfacial bonding strength between carbon fibers and the resin matrix. Conventional sizing agents mostly rely on physical adsorption, and their interfacial shear strength (IFSS) is generally at the level of 50-60 MPa, which is difficult to meet the load-bearing requirements of high-end structural components. At the same time, many existing sizing agents use a large amount of small-molecule emulsifiers to achieve emulsion stability, but these emulsifiers do not participate in the crosslinking reaction and are prone to migrate to the fiber surface later, leading to increased hygroscopicity and decreased resistance to damp heat aging.

[0006] In summary, there is an urgent need in this field to develop a sizing agent for carbon fibers that can impart good bundle properties to carbon fiber tows, facilitate uniform yarn spreading during prepreg preparation, and simultaneously possess excellent interfacial bonding properties and storage stability. Summary of the Invention

[0007] In view of this, the present invention aims to provide a sizing agent for carbon fiber to solve the core technical problem of the difficulty in synergistically optimizing the sizing agent for carbon fiber in terms of bundle bonding and yarn spreading, processing adaptability of ultra-thin prepregs, and interface performance and processing performance.

[0008] To achieve the above objectives, the technical solution created by this invention is implemented as follows: In a first aspect, the present invention provides a sizing agent for carbon fiber, comprising, by weight, 20-40 parts by weight of a main material and deionized water to make up to 100 parts by weight; the main material is composed of the following components by weight percentage: 70%-78% epoxy resin composition, 10%-14% modified polyester emulsifier, 10%-14% functional interface modifier, and 1%-3% auxiliary additives, the sum of the weight percentages of the above components being 100%.

[0009] Furthermore, the epoxy resin composition comprises high epoxy value epoxy resin, medium epoxy value epoxy resin, and low epoxy value epoxy resin; wherein: The high epoxy value epoxy resin used is a bisphenol A type epoxy resin with an epoxy value of 0.50~0.54 eq / 100g and a weight-average molecular weight of 380~480. The epoxy resin with the medium epoxy value is a bisphenol A type epoxy resin with an epoxy value of 0.28~0.33 eq / 100g and a weight-average molecular weight of 850~1200. The low epoxy value epoxy resin used is a bisphenol A type epoxy resin with an epoxy value of 0.12~0.16 eq / 100g and a weight-average molecular weight of 1600~2300.

[0010] Furthermore, based on the total mass of the epoxy resin composition as 100%, the mass percentage of the high epoxy value epoxy resin is 15%~25%, the mass percentage of the medium epoxy value epoxy resin is 40%~55%, and the mass percentage of the low epoxy value epoxy resin is 25%~40%.

[0011] Furthermore, the modified polyester emulsifier is a nonionic surfactant with a number average molecular weight of 1500-5000 and a branching degree of 0.5-0.8.

[0012] Furthermore, the functional interface modifier is a hyperbranched dendritic polymer with a molecular weight of 2000-500; the functional interface modifier is added after the emulsion phase inversion is completed and cooled, and dispersed in the sizing agent for carbon fibers in the form of physical blending.

[0013] Furthermore, the auxiliary additive combination includes a wetting agent and a defoamer; the wetting agent is selected from polyether-modified polysiloxane nonionic surfactants, and the defoamer is selected from mineral oil or polyether defoamers.

[0014] Secondly, the present invention provides a method for preparing a sizing agent for carbon fibers, comprising the following steps: S10. Prepare raw materials by adding 20-40 parts by weight of the main material and deionized water to a total of 100 parts by weight; the main material consists of the following components by mass percentage: 70%-78% epoxy resin composition, 10%-14% modified polyester emulsifier, 10%-14% functional interface modifier, and 1%-3% auxiliary additives, the sum of the mass percentages of the above components being 100%; S20. The epoxy resin composition is mixed with the modified polyester emulsifier, heated to melt, and stirred until uniform to obtain a melt premix. S30. Add a portion of the deionized water to the molten premix and prepare an emulsion by reverse emulsification. S40. After the emulsion has completed phase inversion, continue stirring for 10-20 minutes to stabilize the emulsion, and then allow it to cool naturally to 45-55°C. S50. Add the functional interface modifier and auxiliary additive combination at 45~55℃, stir for 25~40min, add the remaining deionized water, and continue stirring for 10~20min to obtain the sizing agent for carbon fiber.

[0015] Furthermore, the epoxy resin composition comprises three bisphenol A type epoxy resins with different epoxy values ​​and weight-average molecular weights, and step S20 includes: S21. After mixing the epoxy resin composition, heat it to 85~95°C, stir at 300~500 rpm for 20~40 minutes until it is completely melted and mixed evenly to obtain epoxy resin premix. S22. Add the modified polyester emulsifier to the epoxy resin premix and stir at 200-300 rpm for 15-25 minutes at 85-95°C to obtain the melt premix.

[0016] Furthermore, the weight of the deionized water added in step S30 is 0.5 to 1.5 times the weight of the epoxy resin premix.

[0017] Thirdly, the present invention provides a carbon fiber, which is obtained by sizing with a carbon fiber sizing agent provided in the embodiments of the present invention or a carbon fiber sizing agent prepared by the preparation method of the embodiments of the present invention, wherein the sizing amount is 1.0%~1.5%.

[0018] Compared with the prior art, the present invention can achieve the following beneficial effects: (1) The carbon fiber sizing agent created in this invention has excellent bundle properties in the carbon fiber bundle state through the synergistic formulation of bisphenol A type epoxy resins with different epoxy values ​​in the epoxy resin composition, the introduction of hyperbranched emulsifiers and the regulation of functional modifiers. At the same time, it can achieve uniform yarn spreading in the prepreg production process. It is particularly suitable for the seamless preparation of 75g / ㎡, 50g / ㎡ and even 30g / ㎡ ultrathin prepregs, and has the characteristics of high interfacial bonding strength, good storage stability and strong process adaptability.

[0019] (2) The epoxy resin composition of the present invention adopts a stepwise compounding design of three bisphenol A type epoxy resins with different epoxy values ​​and weight-average molecular weights, so that the sizing agent for carbon fiber can form a sizing film on the surface of carbon fiber with suitable mechanical properties and surface characteristics.

[0020] (3) This invention introduces an interface modifier into the sizing agent for carbon fibers. Through the chemical bonding of the terminal groups with the carboxyl and hydroxyl groups on the carbon fiber surface and the ring-opening addition with the epoxy groups in the epoxy matrix, a dense covalent bond network is constructed at the fiber-resin interface, which significantly improves the interfacial bonding strength. Tests show that the interlaminar shear strength of the carbon fiber / epoxy resin composite material treated with the sizing agent of this invention exceeds 80 MPa, which is about 10% to 20% higher than that of the existing conventional technology.

[0021] (4) This invention uses a modified polyester emulsifier in combination with a phase inversion emulsification process to obtain a sizing agent emulsion with an average particle size of 100~200nm, a narrow particle size distribution (PDI≤0.25), and a shelf life of more than 6 months at room temperature, without stratification or sedimentation. The preparation process does not use any organic solvents, and the VOC emissions are near zero, meeting the increasingly stringent environmental protection regulations.

[0022] (5) The carbon fiber sizing agent created in this invention is applicable to PAN-based carbon fibers of different specifications (12K, 24K, 48K, etc.). It is applicable to both wet and dry spraying methods. The resin matrix is ​​mainly epoxy thermosetting resin, which has good industrial versatility and application prospects.

[0023] (6) The 24K carbon fiber bundle treated with the carbon fiber sizing agent described in this invention can achieve a yarn width of 9~10mm on a standard prepreg production line. After yarn spreading, the fiber distribution is uniform and seamless. It can stably prepare 75g / ㎡, 50g / ㎡ and 30g / ㎡ ultra-thin high-end prepregs, and the areal density deviation can be controlled within ±2.0%. Attached Figure Description

[0024] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 This is a schematic flowchart illustrating the preparation method of the carbon fiber sizing agent according to an embodiment of the present invention. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only for explaining the invention and do not constitute a limitation thereof. Similar elements in different embodiments are referred to by associated similar element reference numerals. In the following embodiments, many details are described to facilitate a better understanding of the invention. However, those skilled in the art will readily recognize that some features may be omitted in different situations, or may be replaced by other elements, materials, or methods. In some cases, some operations related to the invention are not shown or described in the specification. This is to avoid overwhelming the core parts of the invention with excessive description. For those skilled in the art, detailed description of these related operations is not necessary; they can fully understand the related operations based on the description in the specification and general technical knowledge in the art.

[0026] It should be noted that, unless otherwise specified, the embodiments and features described in this invention can be combined to form various implementations. Furthermore, the order of the steps or actions in the method description can be changed or adjusted in a manner readily apparent to those skilled in the art. Therefore, the various orders in the specification and drawings are merely for the clear description of a particular embodiment and do not imply a mandatory order, unless otherwise stated that a particular order must be followed.

[0027] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," and "counterclockwise," etc., indicating orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on this invention. The term "based on" should be understood as "at least partially based on." Furthermore, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, features defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more, and the term "including" means "including but not limited to." Various embodiments of the present invention may exist in the form of a range; it should be understood that the description in the form of a range is merely for convenience and brevity and should not be construed as a hard limitation on the scope of the invention; therefore, it should be considered that the range description has specifically disclosed all possible subranges and single numerical values ​​within that range; for example, it should be considered that the range description from 1 to 6 has specifically disclosed subranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. Furthermore, whenever a numerical range is referred to herein, it means including any referenced number (fraction or integer) within the range referred to.

[0028] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art will understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0029] The invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0030] Example 1 This embodiment provides a sizing agent for carbon fiber, which, by weight, includes 20-40 parts by weight of main material and deionized water to make up to 100 parts by weight; the main material is composed of the following components by weight percentage: 70%-78% epoxy resin composition, 10%-14% modified polyester emulsifier, 10%-14% functional interface modifier, and 1%-3% auxiliary additives, the sum of the weight percentages of the above components being 100%.

[0031] In other words, the sizing agent for carbon fiber is composed of the following components in parts by weight: 14 to 31.2 parts by weight of epoxy resin composition, 2 to 5.6 parts by weight of modified polyester emulsifier, 2 to 5.6 parts by weight of functional interface modifier, 0.2 to 1.2 parts by weight of auxiliary additives, and deionized water to make up to 100 parts by weight.

[0032] In this embodiment, the epoxy resin composition comprises three bisphenol A type epoxy resins with different epoxy values ​​and weight-average molecular weights. Based on their epoxy values, they are designated as high-epoxy-value epoxy resin, medium-epoxy-value epoxy resin, and low-epoxy-value epoxy resin, respectively. Their specific parameters are as follows: (1) High epoxy value epoxy resin: epoxy value is 0.50~0.54 eq / 100g, weight average molecular weight is 380~480, accounting for 15%~25% of the total mass of epoxy resin composition. This component provides high reactivity and high crosslinking density, and is the core skeleton for building the mechanical strength of the sizing layer.

[0033] (2) Medium epoxy resin: epoxy value is 0.28~0.33 eq / 100g, weight average molecular weight is 850~1200, accounting for 40%~55% of the total mass of epoxy resin composition. This component balances rigidity and flexibility and is the key factor in regulating bundle properties and yarn unfolding properties.

[0034] (3) Low epoxy value epoxy resin: epoxy value is 0.12~0.16 eq / 100g, weight average molecular weight is 1600~2300, accounting for 25%~40% of the total mass of epoxy resin composition. This component gives the sizing layer good creep resistance and dimensional stability, while reducing the internal stress after film formation.

[0035] Preferably, the sum of the mass percentages of the high epoxy value epoxy resin, medium epoxy value epoxy resin, and low epoxy value epoxy resin is 100%.

[0036] Through the synergistic effect of the proportions of three bisphenol A type epoxy resins with different epoxy values ​​and weight-average molecular weights in this epoxy resin composition, a functional gradient distribution is formed, ranging from high reactivity to good flexibility and then to dimensional stability. In this design, the gradient spans of high, medium, and low epoxy values ​​are relatively large (0.50~0.54, 0.28~0.33, 0.12~0.16), which is not a simple conventional range selection, but a technical solution that can produce synergistic effects determined after multi-dimensional experimental optimization.

[0037] Furthermore, the modified polyester emulsifier is a nonionic surfactant with a number-average molecular weight of 1500-5000, a branching degree of 0.5-0.8, and a hydroxyl value of 100-300 mgKOH / g. The hyperbranched molecular structure of this emulsifier contains a large number of voids, which can significantly reduce the melt viscosity of the epoxy resin system, making the droplets easier to disperse, refine, and stabilize during the phase-conversion emulsification process, thereby achieving efficient emulsification with a lower emulsifier dosage.

[0038] The aforementioned functional interface modifier is a hyperbranched dendritic polymer with a molecular weight of 2000-500. Its core structure is a polymer macromolecule starting from a multifunctional acid. The working principle of this functional interface modifier is as follows: First, at room temperature, the terminal groups can react with the residual carboxyl groups (-COOH) and hydroxyl groups (-OH) on the carbon fiber surface after anodizing treatment, forming a strong chemical anchor. Second, the terminal groups can undergo ring-opening addition reactions with the epoxy groups in the subsequently impregnated epoxy resin matrix, constructing a covalently linked "dendritic anchoring network layer" at the fiber-resin interface, achieving multi-site synergistic anchoring and significantly improving the interfacial shear strength. In this embodiment, the auxiliary additive combination includes a wetting agent and a defoamer; the wetting agent is a polyether-modified polysiloxane nonionic surfactant, and the defoamer is a mineral oil-based or polyether-based defoamer.

[0039] The carbon fiber sizing agent provided by this invention has an average particle size of 100~200nm, a polydispersity index (PDI) ≤0.25, and a storage stability period of not less than 6 months under normal temperature conditions.

[0040] Example 2 This embodiment provides a sizing agent for carbon fiber, comprising 25 parts by weight of the main material and 75 parts by weight of deionized water. The components of the main material are the same as in Example 1, and will not be repeated here. The difference is that, in this embodiment, the mass percentages of each component in the main material are as follows: Epoxy resin composition: 75% (i.e., 18.75 parts); Modified polyester emulsifier: 12% (3.0 parts); Functional interface modifier: 11% (2.75 parts); Auxiliary additive combination (wetting agent 0.4 parts + defoamer 0.1 parts): 2% (0.5 parts).

[0041] The epoxy resin composition contains three types of bisphenol A epoxy resins in the following mass ratio: High epoxy value epoxy resin (epoxy value 0.52 eq / 100g, weight average molecular weight approximately 420): 20%; Medium epoxy value epoxy resin (epoxy value 0.30 eq / 100g, weight average molecular weight approximately 1050): 50%; Low epoxy value epoxy resin (epoxy value 0.14 eq / 100g, weight average molecular weight approximately 1950): 30%.

[0042] Example 3 This embodiment provides a sizing agent for carbon fiber, comprising 30 parts by weight of the main material and 70 parts by weight of deionized water. The components of the main material are the same as in Example 1, and will not be repeated here. The difference is that, in this embodiment, the mass percentages of each component in the main material are as follows: Epoxy resin composition: 77% (i.e., 23.10 parts); Modified polyester emulsifier: 10% (3.0 parts); Functional interface modifier: 10% (3.0 parts); Auxiliary additive combination (wetting agent 0.6 parts + defoamer 0.3 parts): 3% (0.9 parts).

[0043] The epoxy resin composition contains three types of bisphenol A epoxy resins in the following mass ratio: High epoxy value epoxy resin (epoxy value 0.52 eq / 100g, weight average molecular weight approximately 420): 20%; Medium epoxy value epoxy resin (epoxy value 0.30 eq / 100g, weight average molecular weight approximately 1050): 50%; Low epoxy value epoxy resin (epoxy value 0.14 eq / 100g, weight average molecular weight approximately 1950): 30%.

[0044] Example 4 This embodiment provides a sizing agent for carbon fiber, comprising 20 parts by weight of the main material and 80 parts by weight of deionized water. The components of the main material are the same as in Example 1, and will not be repeated here. The difference is that, in this embodiment, the mass percentages of each component in the main material are as follows: Epoxy resin composition: 71% (i.e., 14.2 parts); Modified polyester emulsifier: 14% (2.8 parts); Functional interface modifier: 14% (2.8 parts); Auxiliary additive combination (wetting agent 0.15 parts + defoamer 0.05 parts): 1% (0.2 parts).

[0045] The mass ratio of the three bisphenol A type epoxy resins in the epoxy resin composition is as follows: High epoxy value epoxy resin (epoxy value 0.52 eq / 100g, weight average molecular weight approximately 420): 18%; Medium epoxy value epoxy resin (epoxy value 0.30 eq / 100g, weight average molecular weight approximately 1050): 52%; Low epoxy value epoxy resin (epoxy value 0.14 eq / 100g, weight average molecular weight approximately 1950): 30%.

[0046] Example 5 like Figure 1 As shown, this embodiment provides a method for preparing a sizing agent for carbon fiber. This method can be used to prepare the sizing agent for carbon fiber provided in any of the embodiments 1 to 4 above. The preparation method includes the following steps: S10. Prepare raw materials by adding 20-40 parts by weight of the main material and deionized water to a total of 100 parts by weight. The main material contains the following components by mass percentage: 72%-78% epoxy resin composition, 10%-14% modified polyester emulsifier, 10%-12% functional interface modifier, and 1%-3% auxiliary additives. The sum of the mass percentages of the above components is 100%. The above components are consistent with the components of the main material provided in Example 1, and will not be repeated here.

[0047] S20. Mix the epoxy resin composition with the modified polyester emulsifier, heat to melt, and stir until uniform to obtain a melt premix. More specifically, the epoxy resin composition consists of three bisphenol A type epoxy resins with different epoxy values ​​and weight-average molecular weights, namely, a high epoxy value epoxy resin, a medium epoxy value epoxy resin, and a low epoxy value epoxy resin (consistent with the epoxy resin composition provided in Example 1 above). Step S20 may include: S21. After mixing the epoxy resin composition, heat it to 85~95℃, stir at 300~500rpm for 20~40min until it is completely melted and mixed evenly to obtain epoxy resin premix. Preferably, three epoxy resins with high epoxy value, medium epoxy value and low epoxy value are put into a reaction vessel, heated to 90°C, and stirred at 400 rpm for 30 minutes until completely melted and mixed evenly.

[0048] S22. Add the modified polyester emulsifier to the epoxy resin premix and stir at 200-300 rpm for 15-25 minutes at 85-95℃ to uniformly disperse the modified polyester emulsifier in the epoxy resin premix and obtain the melt premix.

[0049] In practice, the mixture can be stirred at 250 rpm for 20 minutes at 90°C.

[0050] S30. Add some deionized water to the molten premix and prepare an emulsion by reverse emulsification. Specifically, under high-speed shear conditions of 1000-1500 rpm (e.g., 1200 rpm), deionized water is slowly added in batches at a rate of 5-15 parts by weight per minute (preferably 8-12 parts by weight per minute) to the melt premix obtained in step S20, while strictly controlling the temperature of the melt premix to be maintained at 75-85°C, until the system undergoes phase inversion to form a homogeneous emulsion. The weight of the deionized water added in step S30 is 0.5-1.5 times, for example, 1 times, the weight of the epoxy resin premix.

[0051] In this embodiment, the high-speed shearing conditions during the phase-conversion emulsification process and the batch-addition of deionized water ensured the uniform particle size distribution and long-term storage stability of the emulsion.

[0052] S40. After the emulsion phase inversion is complete, continue stirring at 500~800 rpm for 10~20 min to stabilize the emulsion, and then let the system temperature cool down naturally to 45~55℃. S50, add the functional interface modifier and auxiliary additive combination in sequence, stir at 200-400 rpm for 25-40 minutes at 45-55℃ to ensure that the components are fully mixed; add the remaining deionized water (i.e., add deionized water to make the total weight 100 parts by weight), and continue stirring for 10-20 minutes to obtain the sizing agent for carbon fiber.

[0053] Preferably, after continuing to stir for 10 to 20 minutes, the mixture is filtered through a 200 to 400 mesh (e.g., 300 mesh) screen to remove any undispersed particles or impurities that may be present.

[0054] This invention employs a step-by-step process route of "melt premixing - phase inversion emulsification - cooling followed by the addition of interface modifiers and auxiliary additives." Unlike conventional processes that involve "adding all components at once followed by emulsification" or "adding modifiers at high temperature followed by emulsification," this invention adds the functional interface modifier after the emulsion phase inversion is complete and the mixture has cooled down. This effectively avoids premature reaction between the terminal groups and the epoxy groups in the epoxy resin under high-temperature conditions, ensuring that the modifier can perform its chemical anchoring function on the fiber surface.

[0055] Example 6 This embodiment provides a carbon fiber, which is obtained by sizing with the carbon fiber sizing agent provided in Embodiment 1 of the present invention or the carbon fiber sizing agent prepared by the preparation method in Embodiment 5, with a sizing amount of 1.0%~1.5%. The formula for calculating the sizing amount (also commonly referred to as sizing rate or sizing content) is as follows: ; in, This refers to the absolute dry mass of unsized carbon fiber. The total mass of carbon fiber after sizing and complete drying (removal of moisture and solvent).

[0056] In this embodiment, the carbon fiber is PAN-based carbon fiber, and the tow specifications are 12K, 24K, or 48K. During the sizing process, the carbon fiber (e.g., 24K PAN-based carbon fiber) is coated with a sizing agent using an impregnation method. The temperature of the sizing bath is controlled at 25-30°C, the impregnation time at 6-9 seconds, the gap between the extrusion rollers at 0.12-0.16 mm, and the sizing amount is controlled at 1.10%-1.40% (determined by thermogravimetric analysis). The drying conditions are 150-190°C, hot air drying for 60 seconds.

[0057] The carbon fiber sizing agent provided in Example 2 was used below to investigate the effect of sizing amount on sizing effect by adjusting the extrusion roller gap and immersion time to achieve different sizing amounts. Other sizing parameters included: sizing tank temperature 25-30℃, drying temperature 150-190℃, and drying time 60 seconds. The specific extrusion roller gap, immersion time, and sizing amount are shown in Table 1 below. Table 1

[0058] Tests showed that the 24K carbon fiber tows treated with the three sizing methods all achieved a spread width of 9.0~9.5mm, with uniform spread and no obvious gaps or fuzz.

[0059] After sizing, the 24K carbon fiber tow has a spreading width of 9~10mm on the prepreg production line, which can be used to prepare ultra-thin prepregs of 75g / ㎡, 50g / ㎡ or 30g / ㎡, and the spreading is uniform and seamless.

[0060] The following uses 24K carbon fiber treated with sizing agent (Example 2) as reinforcement and bisphenol A epoxy resin as matrix resin to produce prepregs of different areal densities on a standard prepreg production line. The following process parameters were controlled during prepreg preparation: impregnation temperature 40-50℃, resin viscosity controlled at 1500-2500 cps, yarn tension 250-300 cN, and hot-pressing temperature 60-80℃ (first stage) and 90-110℃ (second stage). The test results are shown in Table 2 below. Table 2

[0061] Test results show that 24K carbon fiber tow treated with the carbon fiber sizing agent provided by this invention can stably achieve seamless preparation of 75g / ㎡, 50g / ㎡ and 30g / ㎡ ultrathin prepregs. Among them, the areal density deviation of the 30g / ㎡ ultrathin prepreg is controlled within ±2.1%, which meets the quality control requirements of high-end prepreg products.

[0062] Example 7 To fully verify the superiority of this invention, comparative experiments were conducted using comparative examples. All comparative examples used 24K PAN-based carbon fiber and the same sizing process parameters, with the sizing amount controlled within the range of 1.20% to 1.30%. The aforementioned sizing process parameters included: sizing tank temperature 25-30℃, drying temperature 150-190℃, drying time 60 seconds, impregnation time 6-9 seconds, and extrusion roller gap 0.12-0.16 mm. The test results are shown in Table 3 below: Table 3

[0063] In Comparative Example 1, a commercially available epoxy carbon fiber sizing agent from a mainstream domestic brand was selected, and 24K carbon fiber was sized according to the sizing process recommended in the product instructions.

[0064] Test results show that the 24K carbon fiber tow has a spread width of approximately 7.0~8.5mm, and the fiber distribution uniformity after spreading is poor, with obvious gaps in some areas. The areal density deviation of the carbon fiber treated with this epoxy-based carbon fiber sizing agent reached ±3%~5% when preparing 75g / ㎡ prepreg, and it was unable to stably prepare prepregs with areal densities of 50g / ㎡ and below, with an interlaminar shear strength of approximately 60MPa.

[0065] Comparative Example 2 used the same formulation as Example 2, but replaced the epoxy resin composition with a single medium-epoxy value epoxy resin (epoxy value 0.30 eq / 100g, weight-average molecular weight approximately 1050), and the amount used was the same as the total amount of epoxy resin composition used in Example 2. Other components and proportions were the same as in Example 2, and the preparation method was the same.

[0066] More specifically, Comparative Example 2 used an epoxy-based carbon fiber sizing agent, which, by weight, comprised 25 parts by weight of the main material and 75 parts by weight of deionized water. The mass percentages of each component in the main material are as follows: Medium epoxy value epoxy resin: 75% (i.e., 18.75 parts); Modified polyester emulsifier: 12% (3.0 parts); Functional interface modifier: 11% (2.75 parts); Auxiliary additive combination (wetting agent 0.4 parts + defoamer 0.1 parts): 2% (0.5 parts).

[0067] Test results showed that the 24K carbon fiber tow treated with the epoxy-based carbon fiber sizing agent exhibited significantly insufficient bundle cohesion during storage and transportation, showing some degree of divergence and fuzzing. The yarn width ranged from 7.5 to 9.0 mm, with large fluctuations and poorer uniformity than in Example 2. The 30 g / m² ultrathin prepreg could not be stably prepared, resulting in multiple localized gaps after yarn unfolding. The interlaminar shear strength was approximately 62 MPa.

[0068] Comparative Example 3 used the same formulation as Example 2, but without the addition of a functional interface modifier (the original proportion of the functional interface modifier was supplemented by the epoxy resin composition). Other components and proportions were the same as in Example 2, and the preparation method was the same.

[0069] More specifically, Comparative Example 3 used an epoxy-based carbon fiber sizing agent, which, by weight, comprised 25 parts by weight of the main material and 75 parts by weight of deionized water. The mass percentages of each component in the main material are as follows: Epoxy resin composition: 86% (i.e., 21.5 parts); Modified polyester emulsifier: 12% (3.0 parts); Auxiliary additive combination (wetting agent 0.4 parts + defoamer 0.1 parts): 2% (0.5 parts).

[0070] The epoxy resin composition contains three types of bisphenol A epoxy resins in the following mass ratio: High epoxy value epoxy resin (epoxy value 0.52 eq / 100g, weight average molecular weight approximately 420): 20%; Medium epoxy value epoxy resin (epoxy value 0.30 eq / 100g, weight average molecular weight approximately 1050): 50%; Low epoxy value epoxy resin (epoxy value 0.14 eq / 100g, weight average molecular weight approximately 1950): 30%.

[0071] Test results showed that the interlaminar shear strength was approximately 63 MPa, significantly lower than that of Example 2. The yarn unfolding width and prepreg areal density were essentially the same as in Example 2. This comparison indicates that the interface modifier plays a crucial role in improving interfacial chemical bonding and the mechanical properties of the composite material, while having no negative impact on the yarn unfolding performance of the sizing agent.

[0072] The test methods for some of the test items in Table 3 above are as follows: (1) Determination of average particle size (nm) of the emulsion: The average particle size of the emulsion was determined using a dynamic light scattering (DLS) particle size analyzer. Before testing, the sizing agent emulsion was diluted with deionized water to a solid content of 0.10±0.01wt%, and then kept at a constant temperature of 25±0.5℃ for 5 minutes before measurement. Three parallel samples were prepared for each sample, and each parallel sample was measured three times consecutively. The average value of all measured data was taken as the average particle size of the emulsion for that sample, recorded in nanometers (nm).

[0073] (2) PDI value determination: The PDI value of the emulsion was determined using a dynamic light scattering particle size analyzer. Before testing, the sizing agent emulsion was diluted with deionized water to a solid content of 0.10±0.01wt%, and then kept at a constant temperature of 25±0.5℃ for 5 minutes before measurement. The PDI value was automatically calculated and output by the instrument software. Three parallel samples were prepared for each sample, and each parallel sample was measured three times consecutively. The average value of all measured data was taken as the PDI value of the sample.

[0074] (3) Determination of the stability period (in months) at room temperature: Take 100 mL of the sizing agent emulsion and place it in a covered transparent glass sample bottle. Store it at 25±2℃. Take samples every 7 days from the date of storage and record whether the sample shows stratification, precipitation, gelation or discoloration. At the same time, monitor the change in the average particle size of the emulsion according to the particle size determination method described above. The earliest time when obvious stratification, precipitation or average particle size increase exceeds 20% of the initial particle size is taken as the stability period of the emulsion at room temperature, and recorded in months. If none of the above phenomena occur after 6 months of storage, it is recorded as ">6 months".

[0075] (4) Measurement of 24K fiber bundle width (mm): Take a 1m long 24K carbon fiber bundle and lay it flat on a black velvet cloth without additional tension. Use a digital vernier caliper to select 10 points at equal intervals along the length of the bundle and measure the natural width of the bundle at each point. Take the arithmetic mean of the 10 measurements as the bundle width of the sample and record it in millimeters (mm).

[0076] (5) Determination of deviation (%) of 75g / m² prepreg: Take one sample from the left, center, and right sides of the width direction of the 75g / m² prepreg roll, and select at least 3 different positions along the length direction. Cut 100mm×100mm samples using a standard cutter, accurately weigh each sample, and calculate the areal density (g / m²) of each sample. The areal density deviation is calculated according to the following formula: Deviation (%) = |Measured areal density Target areal density (75 g / m²) | / Target areal density (75 g / m²) × 100%. Take the average deviation of all samples as the areal density deviation of this batch of prepreg.

[0077] (6) Determination of the yarn spreading quality of 50g / ㎡ prepreg: The 50g / ㎡ prepreg is spread through a yarn spreading device. After the yarn spreading is completed, the fiber bundles after the yarn spreading are inspected by visual observation combined with an optical magnifying glass. The degree of spreading and uniformity of the fiber bundles after the yarn spreading are observed, and the presence of defects such as yarn bundling, missing yarn, resin accumulation or exposed fibers is recorded.

[0078] (7) Test of the feasibility of 30 g / m² prepreg: The target areal density was set at 30 g / m² on the actual production line for prepreg preparation. During the evaluation, the uniformity of resin coating, the state of fiber bundle expansion and demolding were recorded. The presence of dry yarn, resin voids or local accumulation on the surface of the prepreg was observed. The process stability during continuous production was evaluated. The ability to continuously and stably prepare 30 g / m² prepreg with uniform surface and no obvious defects was used as the criterion for "feasibility".

[0079] (8) Determination of interlaminar shear strength (MPa): The interlaminar shear strength of the composite material was determined by the short beam shear method in accordance with GB / T 30969-2014 "Test Method for Shear Properties of Short Beams of Polymer-Based Composite Materials". After sizing, carbon fibers were prepared into unidirectional composite laminates according to the specified process. Specimens of the specified size were cut along the fiber direction and subjected to three-point bending loading on a universal testing machine. The maximum failure load was recorded, and the interlaminar shear strength was calculated according to the formula specified in the standard, recorded in megapascals (MPa). At least five valid specimens were tested under each condition, and the arithmetic mean was taken as the final result.

[0080] As can be seen from the test results in Table 3 above: (1) In terms of yarn spreading performance, the yarn spreading width of Examples 2-4 was consistently 9.0-9.5 mm, which is much higher than that of Comparative Example 1 (commercially available product) (7.0-8.5 mm), and the yarn spreading uniformity was significantly better than all comparative examples. This invention precisely controls the mechanical properties of the sizing film through a tiered compounding design of three epoxy resins, achieving a perfect balance between bundle bonding and yarn spreading performance. This is the key to the seamless preparation of 30 g / m² ultrathin prepreg.

[0081] (2) In terms of interface performance, the interlaminar shear strength index (ILSS≥80MPa) of Examples 2-4 far exceeds that of all comparative examples.

[0082] (3) Regarding emulsion stability, Examples 2-4 achieved storage stability of ≥6 months using only a single hyperbranched emulsifier, indicating that the emulsification efficiency and particle size control capability of the hyperbranched emulsifier in this invention are far superior to conventional emulsifiers.

[0083] It should be understood that the various forms of processes shown above can be used to reorder, add, or delete steps. For example, the steps described in this invention disclosure can be executed in parallel, sequentially, or in different orders, as long as the desired result of the technical solution disclosed in this invention can be achieved, and this is not limited herein.

[0084] The specific embodiments described above do not constitute a limitation on the scope of protection of this invention. Those skilled in the art should understand that various modifications, combinations, sub-combinations, and substitutions can be made according to design requirements and other factors. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this invention should be included within the scope of protection of this invention.

Claims

1. A sizing agent for carbon fibers, characterized by: The product comprises 20-40 parts by weight of main material and deionized water to make up to 100 parts by weight; the main material is composed of the following components by weight percentage: 70%-78% epoxy resin composition, 10%-14% modified polyester emulsifier, 10%-14% functional interface modifier, and 1%-3% auxiliary additives, the sum of the weight percentages of the above components being 100%.

2. The sizing agent for carbon fibers according to claim 1, characterized by: The epoxy resin composition comprises high epoxy value epoxy resin, medium epoxy value epoxy resin, and low epoxy value epoxy resin; wherein: The high epoxy value epoxy resin used is a bisphenol A type epoxy resin with an epoxy value of 0.50~0.54 eq / 100g and a weight-average molecular weight of 380~480. The epoxy resin with the medium epoxy value is a bisphenol A type epoxy resin with an epoxy value of 0.28~0.33 eq / 100g and a weight-average molecular weight of 850~1200. The low epoxy value epoxy resin used is a bisphenol A type epoxy resin with an epoxy value of 0.12~0.16 eq / 100g and a weight-average molecular weight of 1600~2300.

3. The sizing agent for carbon fiber according to claim 2, characterized in that: Based on the total mass of the epoxy resin composition as 100%, the mass percentage of the high epoxy value epoxy resin is 15%~25%, the mass percentage of the medium epoxy value epoxy resin is 40%~55%, and the mass percentage of the low epoxy value epoxy resin is 25%~40%.

4. The sizing agent for carbon fiber according to claim 1, characterized in that, The modified polyester emulsifier is a nonionic surfactant with a number average molecular weight of 1500-5000 and a branching degree of 0.5-0.

8.

5. The sizing agent for carbon fiber according to claim 1, characterized in that, The functional interface modifier is a hyperbranched dendritic polymer with a molecular weight of 2000-500. The functional interface modifier is added after the emulsion phase inversion is completed and cooled down, and dispersed in the sizing agent for carbon fiber in the form of physical blending.

6. The sizing agent for carbon fiber according to claim 1, characterized in that, The auxiliary additive combination includes a wetting agent and a defoamer; the wetting agent is selected from polyether-modified polysiloxane nonionic surfactants, and the defoamer is selected from mineral oil or polyether defoamers.

7. A method for preparing a sizing agent for carbon fiber, characterized in that, Including the following steps: S10. Prepare raw materials by adding 20-40 parts by weight of the main material and deionized water to a total of 100 parts by weight; the main material consists of the following components by mass percentage: 70%-78% epoxy resin composition, 10%-14% modified polyester emulsifier, 10%-14% functional interface modifier, and 1%-3% auxiliary additives, the sum of the mass percentages of the above components being 100%; S20. The epoxy resin composition is mixed with the modified polyester emulsifier, heated to melt, and stirred until uniform to obtain a melt premix. S30. Add a portion of the deionized water to the molten premix and prepare an emulsion by reverse emulsification. S40. After the emulsion has completed phase inversion, continue stirring for 10-20 minutes to stabilize the emulsion, and then allow it to cool naturally to 45-55°C. S50. Add the functional interface modifier and auxiliary additive combination at 45~55℃, stir for 25~40min, add the remaining deionized water, and continue stirring for 10~20min to obtain the sizing agent for carbon fiber.

8. The method for preparing the sizing agent for carbon fiber according to claim 7, characterized in that, The epoxy resin composition comprises three bisphenol A type epoxy resins with different epoxy values ​​and weight-average molecular weights, and step S20 includes: S21. After mixing the epoxy resin composition, heat it to 85~95°C, stir at 300~500 rpm for 20~40 minutes until it is completely melted and mixed evenly to obtain epoxy resin premix. S22. Add the modified polyester emulsifier to the epoxy resin premix and stir at 200-300 rpm for 15-25 minutes at 85-95°C to obtain the melt premix.

9. The method for preparing the sizing agent for carbon fiber according to claim 8, characterized in that, The weight of the deionized water added in step S30 is 0.5 to 1.5 times the weight of the epoxy resin premix.

10. A carbon fiber, characterized in that, The carbon fiber is obtained by sizing with the carbon fiber sizing agent according to any one of claims 1 to 6 or the carbon fiber sizing agent prepared by the preparation method according to any one of claims 7 to 8, and the sizing amount is 1.0% to 1.5%.

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