STABILITY ADJUSTMENT METHOD FOR CARBON FIBER EPOXY RESIN SIZING AGENT

Description

TECHNICAL FIELD

The disclosure relates to the technical field of polymer materials, and more particularly to a stability adjustment method for a carbon fiber epoxy resin sizing agent.

BACKGROUND

Carbon fiber refers to a high strength and high modulus inorganic fiber material made from polyacrylonitrile and viscose fibers as raw materials through high-temperature oxidation and carbonization, with a carbon content above 90%. Heat resistance of the carbon fiber ranks first among all chemical fibers, which is an excellent material for manufacturing high-tech equipment such as aerospace vehicles. However, due to its inherent brittleness, a large number of broken filaments and surface grooves are generated during production, transportation, and subsequent processing. This not only reduces the strength of the carbon fiber itself but also affects the comprehensive mechanical properties of carbon fiber-reinforced composites.

Carbon fiber-reinforced thermoset resin composites exhibit excellent characteristics such as high specific strength, high specific modulus, corrosion resistance, and light weight, making them widely used in aerospace, sporting goods, and many other fields. The most commonly used thermosetting resin for carbon fiber-reinforced resins is epoxy resin. The mechanical properties of the carbon fiber-reinforced epoxy resin composites are mainly determined by the mechanical properties of the reinforcement (carbon fiber) and the epoxy resin matrix themselves, as well as the interfacial bonding performance between the carbon fiber and the epoxy resin matrix. Therefore, sizing treatment of the fibers is necessary to improve the poor interfacial bonding between the carbon fiber and the epoxy resin matrix.

The sizing process is the best surface modification method for improving the interfacial properties of the carbon fiber. A carbon fiber epoxy resin sizing agent can fill many grooves on the carbon fiber surface, thereby improving the strength of the carbon fiber itself. The carbon fiber epoxy resin sizing agent also increases the bundling properties and wettability of the carbon fiber, thereby enhancing the interface between the carbon fiber and the epoxy resin matrix, and improving the interlaminar shear strength and interfacial shear strength of the composite material.

Currently, the carbon fiber epoxy resin sizing agent is predominantly emulsion-type sizing agent, which is water-soluble sizing agent formed by adding surfactants and other auxiliaries to the main polymer, and using water as the dispersant. The carbon fiber epoxy resin sizing agent offers advantages such as low pollution, low cost, and low residue.

The stability of a carbon fiber epoxy resin sizing agent is an important performance indicator. Van der Waals forces (attractive forces) exist between latex particles, which cause the liquid particles to attract each other and coalesce spontaneously. Generally, the smaller the emulsion particle size, the better the stability of the emulsion, and the better the sizing uniformity. Conversely, when the emulsion particle size is too large, it not only affects the emulsion stability but also, due to the large particles not easily penetrating between fiber tows, affects the uniformity of sizing. Furthermore, a higher zeta potential means the emulsion particles carry similar charges, increasing mutual repulsion. When the repulsive force between particles is greater than the van der Waals force, the droplets are less likely to contact each other, thereby reducing the possibility of coalescence and thus improving emulsion stability.

Therefore, how to improve the stability of the carbon fiber epoxy resin sizing agent is a problem urgently needing a solution by those skilled in the art.

SUMMARY

In view of this, a purpose of the disclosure is to provide a stability adjustment method for a carbon fiber epoxy resin sizing agent to solve disadvantages in the related art.

In order to achieve the above purpose, the disclosure adopts the following technical solution.

A stability adjustment method for a carbon fiber epoxy resin sizing agent includes the following steps: adding a nonionic surfactant to the carbon fiber epoxy resin sizing agent until a hydrophile-lipophile balance (HLB) value of the system (i.e., a mixture of the nonionic surfactant and the carbon fiber epoxy resin sizing agent) is 9.67-13.30, followed by ultrasonic dispersion, thereby obtaining a highly dispersed and stable carbon fiber epoxy resin sizing agent.

In an embodiment, the nonionic surfactant is at least one selected from the group consisting of sorbitan monopalmitate 40 (i.e., SPAN® 40), polysorbate 40 (i.e., TWEEN® 40), polysorbate 80 (i.e., TWEEN® 80) and polysorbate 85 (i.e., TWEEN® 85).

In an embodiment, the nonionic surfactant is the polysorbate 80 and the polysorbate 85 in a mass ratio of 1:2; or the nonionic surfactant is the polysorbate 40 and the polysorbate 85 in a mass ratio of 1:1; or the nonionic surfactant is the sorbitan monopalmitate 40 and the polysorbate 40 in a mass ratio of 2:1; or the nonionic surfactant is the sorbitan monopalmitate 40 and the polysorbate 80 in a mass ratio of 2:3; or the nonionic surfactant is the sorbitan monopalmitate 40 and the polysorbate 85 in a mass ratio of 1:4.

In an embodiment, an amount of the nonionic surfactant is 1% of a weight of the carbon fiber epoxy resin sizing agent.

In an embodiment, a time for the ultrasonic dispersion is in a range of 0.5 minutes (min) to 3 min. In an embodiment, the time is 2 min.

In an embodiment, the carbon fiber epoxy resin sizing agent comprises the following raw materials: polyethylene glycol, phthalic anhydride, and epoxy resin, in a molar ratio of 1:1:1.

In an embodiment, the polyethylene glycol is polyethylene glycol 1000.

The beneficial effects of adopting the aforementioned further technical solution lies in that, the polyethylene glycol 1000 (PEG 1000) is a mixture formed by polycondensation of ethylene oxide and water. A molecular formula of the polyethylene glycol 1000 is HO(CH₂CH₂O)_nH, where n represents an average number of ethylene oxide groups.

In an embodiment, the epoxy resin includes epoxy resin E51 and epoxy resin E44.

The beneficial effects of adopting the aforementioned further technical solution lies in that, the epoxy resin E51 is a type of thermosetting polymer synthetic material with excellent properties such as good adhesion, corrosion resistance, insulation, and high strength. Its product features a high epoxy value, low viscosity, and light color. The epoxy resin E44 is a bisphenol A-type liquid epoxy resin, belonging to a category of basic epoxy resins. The epoxy resin E44 is low in toxicity and appears as a colorless or pale yellow viscous liquid.

In an embodiment, a mass ratio of the epoxy resin E51 and the epoxy resin E44 is 2:1.

The beneficial effect of adopting the aforementioned further technical solution lies in that, based on an epoxy equivalent of the epoxy resin E51 being 190 and that of the epoxy resin E44 being 220, an epoxy equivalent of the compounded epoxy resin is calculated as 200.

In an embodiment, a preparation method of the carbon fiber epoxy resin sizing agent comprises:

• • S1, weighing the raw materials according to the molar ratio of the raw materials in the carbon fiber epoxy resin sizing agent;
  • S2, performing reduced pressure distillation on the polyethylene glycol to obtain anhydrous polyethylene glycol;
  • S3, mixing the anhydrous polyethylene glycol and the phthalic anhydride to obtain a first mixture, and adding a catalyst A into the first mixture for reaction to obtain phthalic anhydride-modified polyethylene glycol; and
  • S4, mixing the phthalic anhydride-modified polyethylene glycol and the epoxy resin to obtain a second mixture, and adding a catalyst B into the second mixture for reaction to obtain the carbon fiber epoxy resin sizing agent.

In an embodiment, in step S3, the catalyst A is p-toluenesulfonic acid, the reaction is performed under nitrogen protection at a temperature of 120 Celsius degrees (° C.) for 2.5 hours (h). In step S4, the catalyst B is potassium persulfate, the reaction is performed under nitrogen protection at a temperature of 125° C. for 4 h.

The beneficial effect of adopting the aforementioned further technical solution lies in that, the disclosure utilizes the phthalic anhydride-modified polyethylene glycol as a modifier for grafting with the epoxy resin. Due to a significant steric hindrance of a molecular structure of the phthalic anhydride, it ensures that the reaction with the epoxy resin predominantly proceeds via grafting at one end. This facilitates the preparation of a water-soluble carbon fiber epoxy resin sizing agent that is hydrophobic at one end, hydrophilic at the other end, and possesses self-emulsifying properties.

As can be seen from the above technical solutions, compared with the related art, the disclosure provides the following beneficial effects.

1. To meet the application requirements for the stability of the carbon fiber epoxy resin sizing agent, it is necessary to ensure that the prepared sizing agent has a relatively small emulsion particle size while also increasing a zeta potential of the carbon fiber epoxy resin sizing agent. This enhances the repulsive force between emulsion particles, thereby reducing the likelihood of coalescence of the emulsion particles. For this purpose, the disclosure adds commonly used surfactants, employing one or two compounded surfactants to increase the zeta potential of the carbon fiber epoxy resin sizing agent emulsion. The improved stability of the carbon fiber epoxy resin sizing agent can reduce the formation of small latex particle sedimentation during transportation and storage, minimize surface defects on carbon fibers after sizing, thereby enhancing the application performance of the carbon fiber-reinforced epoxy resin matrix composites.

2. The disclosure improves the stability of the carbon fiber epoxy resin sizing agent by increasing the zeta potential of the carbon fiber epoxy resin sizing agent. Simultaneously, to adapt to the application scope of various carbon fiber epoxy resin sizing agents with different stability requirements, different surfactants are selected and compounded to meet the needs of the carbon fiber epoxy resin sizing agents with varying particle sizes.

3. The disclosure employs a method of compounding surfactants to adjust the HLB value of the system, so that the prepared carbon fiber epoxy resin sizing agent possesses good stability, dispersibility, and extended storage life. Furthermore, the disclosure investigates the influence of the calculated HLB value of compounded surfactants (i.e., the “composite emulsifier” in FIG. 1) at different mixing ratios on the stability, interfacial bonding, particle size, and other factors affecting carbon fiber surface modification of the sizing agent. By analyzing performance tests such as particle size, zeta potential, scanning electron microscope (SEM), and mechanical stability, the effectiveness of the chemical bonding formed by the carbon fiber epoxy resin sizing agent on the carbon fiber surface is analyzed. The aim is to establish a stable wetting system that modifies the surface of inert carbon fibers, thereby improving the bundling and dispersibility of carbon fiber tows. This part of the research work is illustrated in FIG. 1.

4. The disclosure first adds compounded surfactants to the carbon fiber epoxy resin sizing agent, then utilizes ultrasonic dispersion in an aqueous solution system for 0.5-3 min to obtain a highly dispersed and stable carbon fiber epoxy resin sizing agent (i.e., the “wetting agent” in FIG. 2). This agent is then used for sizing surface-treated carbon fibers via wetting. The sized carbon fibers undergo surface gold sputtering treatment, and finally, field emission scanning electron microscopy is used to observe the effect of carbon fiber surface modification. The influence of the HLB value of the compounded surfactants on the stability of the carbon fiber epoxy resin sizing agent is evaluated by testing the particle size and the zeta potential of the carbon fiber epoxy resin sizing agent. This part of the research work is illustrated in FIG. 2.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a schematic diagram of an emulsification and stabilization process of a carbon fiber epoxy resin sizing agent according to the disclosure.

FIG. 2 illustrates a schematic diagram of a surface wetting modification process of carbon fibers according to the disclosure.

FIG. 3A illustrates a surface SEM image of carbon fiber tows after surface treatment, sizing with the carbon fiber epoxy resin sizing agent from an embodiment 2, and subsequent surface gold sputtering treatment.

FIG. 3B illustrates a surface SEM image of carbon fiber tows after surface treatment, sizing with the carbon fiber epoxy resin sizing agent from an embodiment 3, and subsequent surface gold sputtering treatment.

FIG. 4 illustrates physical photographs of the carbon fiber epoxy resin sizing agents from embodiments 2-6 after storage for 30 days, 60 days, 180 days, and 300 days, respectively.

DETAILED DESCRIPTION OF EMBODIMENTS

The following provides a clear and complete description of technical solutions in embodiments of the disclosure. Apparently, the described embodiments are only some of the embodiments of the disclosure, and not all of the embodiments. Based on the embodiments of the disclosure, all other embodiments obtained by those skilled in the art without creative labor are within a scope of protection of the disclosure.

Embodiment 1

A carbon fiber epoxy resin sizing agent includes the following raw materials: polyethylene glycol 1000, phthalic anhydride, and epoxy resin, in a molar ratio of 1:1:1. Specifically, the epoxy resin is compounded by epoxy resin E51 and epoxy resin E44 according to a mass ratio of 2:1.

A preparation method of the above carbon fiber epoxy resin sizing agent includes the following steps S1-S4.

In S1, the raw materials of the above carbon fiber epoxy resin sizing agent are weighed according to the molar ratio.

In S2, the polyethylene glycol 1000 is placed in a rotary evaporator. Water molecules in the polyethylene glycol 1000 are completely removed by reduced pressure distillation to obtain anhydrous polyethylene glycol 1000.

In S3, the anhydrous polyethylene glycol 1000 and the phthalic anhydride are mixed to obtain a first mixture. The first mixture is placed in a four-neck separable glass reactor equipped with a thermometer, a stirrer, a condenser for reflux, and a nitrogen inlet. Nitrogen protection is provided. P-toluenesulfonic acid is added into the first mixture as a catalyst. The first mixture is heated to 120° C. for reaction for 2.5 h to obtain phthalic anhydride-modified polyethylene glycol.

In S4, the phthalic anhydride-modified polyethylene glycol and the epoxy resin are mixed to obtain a second mixture. The second mixture is placed in the four-neck separable glass reactor equipped with the thermometer, the stirrer, the condenser for reflux, and the nitrogen inlet. Nitrogen protection is provided. Potassium persulfate is added into the second mixture as a catalyst. The second mixture is heated to 125° C. for reaction for 4 h to obtain the carbon fiber epoxy resin sizing agent.

Embodiment 2

A stability adjustment method for a carbon fiber epoxy resin sizing agent includes the following steps. 1 wt % nonionic surfactants compounded by polysorbate 80 (TWEEN® 80) and polysorbate 85 (TWEEN® 85) in a mass ratio of 1:2 are added to the carbon fiber epoxy resin sizing agent prepared in the embodiment 1 until a HLB value of the system is adjusted to 12.30. Ultrasonic dispersion is performed on the system for 1 min to obtain a highly dispersed and stable carbon fiber epoxy resin sizing agent.

Embodiment 3

A stability adjustment method for a carbon fiber epoxy resin sizing agent includes the following steps. 1 wt % nonionic surfactants compounded by polysorbate 40 (TWEEN® 40) and TWEEN® 85 in a mass ratio of 1:1 are added to the carbon fiber epoxy resin sizing agent prepared in the embodiment 1 until a HLB value of the system is adjusted to 13.30. Ultrasonic dispersion is performed on the system for 2 min to obtain a highly dispersed and stable carbon fiber epoxy resin sizing agent.

Embodiment 4

A stability adjustment method for a carbon fiber epoxy resin sizing agent includes the following steps. 1 wt % nonionic surfactants compounded by sorbitan monopalmitate 40 (SPAN® 40) and TWEEN® 40 in a mass ratio of 2:1 are added to the carbon fiber epoxy resin sizing agent prepared in the embodiment 1 until a HLB value of the system is adjusted to 9.67. Ultrasonic dispersion is performed on the system for 3 min to obtain a highly dispersed and stable carbon fiber epoxy resin sizing agent.

Embodiment 5

A stability adjustment method for a carbon fiber epoxy resin sizing agent includes the following steps. 1 wt % nonionic surfactants compounded by SPAN® 40 and TWEEN® 80 in a mass ratio of 2:3 are added to the carbon fiber epoxy resin sizing agent prepared in the embodiment 1 until a HLB value of the system is adjusted to 11.68. Ultrasonic dispersion is performed on the system for 1.5 min to obtain a highly dispersed and stable carbon fiber epoxy resin sizing agent.

Embodiment 6

A stability adjustment method for a carbon fiber epoxy resin sizing agent includes the following steps. 1 wt % nonionic surfactants compounded by SPAN® 40 and TWEEN® 85 in a mass ratio of 1:4 are added to the carbon fiber epoxy resin sizing agent prepared in the embodiment 1 until a HLB value of the system is adjusted to 10.14. Ultrasonic dispersion is performed on the system for 0.5 min to obtain a highly dispersed and stable carbon fiber epoxy resin sizing agent.

Performance Test

1. Particle Size and Zeta Potential Measurement Test

The carbon fiber epoxy resin sizing agent prepared in the embodiment 1 and the highly dispersed and stable carbon fiber epoxy resin sizing agents prepared in the embodiments 2-6 are taken separately. Each sample is diluted with distilled water to a concentration of 50-100 kilo count per second (Kcps) to obtain diluted samples. The particle size and zeta potential of each diluted sample are measured by using a laser particle size analyzer (e.g., model NanoBrook 90plus PALS, purchased from Brookhaven Instruments, USA). The changes in particle size and zeta potential before and after adding the compounded surfactants are compared. The results are shown in Table 1.

As shown in Table 1, compared with the embodiment 1, the values of the Zeta potential for the highly dispersed and stable sizing agents from the embodiments 2-6 exceed 35 after the addition of the compounded surfactants. Furthermore, in terms of the overall trend, a larger HLB value indicates better hydrophilicity, resulting in a smaller emulsion particle size, which is more conducive to improving the stability of the carbon fiber epoxy resin sizing agent.

2. Mechanical Stability Test

The highly dispersed and stable carbon fiber epoxy resin sizing agents prepared in the embodiments 2-6 are taken separately. Each sample is subjected to high-speed centrifugation by using a high-speed benchtop centrifuge (e.g., model TGL-16B, Shanghai Anting Scientific Instrument Factory) at a speed of 3000 revolutions per minute (rpm) for 30 min. The stratification of each sample is observed at 5 min, 10 min, 15 min, 20 min, 25 min, and 30 min during centrifugation. The results are shown in Table 2.

As shown in Table 2, after the addition of the compounded surfactants, the stability of the carbon fiber epoxy resin sizing agents from the embodiments 2-6 is significantly improved. No stratification is observed after high-speed centrifugation at 3000 rpm for 30 min.

3. Sizing Effect Test

The highly dispersed and stable carbon fiber epoxy resin sizing agents prepared in the embodiment 2 and the embodiment 3 are taken. Each sizing agent is uniformly applied to surface-treated T300 carbon fiber tows. The sized carbon fiber tows then undergo surface gold sputtering treatment. The application effect of the carbon fiber epoxy resin sizing agent is evaluated by using a field emission scanning electron microscope (e.g., model JSM-7610F Plus, JEOL, Japan). The results are shown in FIG. 3.

FIG. 3A illustrates a surface SEM image of carbon fiber tows after surface treatment, sizing with the sizing agent from the embodiment 2, and surface gold sputtering treatment. FIG. 3B illustrates a surface SEM image of carbon fiber tows after surface treatment, sizing with the sizing agent from the embodiment 3, and surface gold sputtering treatment.

As can be seen from FIGS. 3A and 3B, the highly dispersed and stable sizing agents from the embodiments 2 and 3 can form a complete coating on the carbon fiber surface. The dispersion effect on the carbon fiber surface is also improved.

4. Storage Stability Test

The highly dispersed and stable carbon fiber epoxy resin sizing agents prepared in the embodiments 2-6 are taken. The samples are stored under static conditions for 300 days. The presence of settled particles at the bottom of each sample is observed after 30 days, 60 days, 180 days, and 300 days of storage. The results are shown in FIG. 4.

In FIG. 4, labels “1”, “2”, “3”, “4”, and “5” correspond to the highly dispersed and stable sizing agents from embodiments 2, 3, 4, 5, and 6, respectively.

As can be seen from FIG. 4, no settled particles are observed at the bottom of the sizing agents from the embodiments 2-6 within 300 days.

The above description of the disclosed embodiments enables those skilled in the art to make or use the disclosure. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the disclosure. Thus, the disclosure is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.