Introducing heterocycles into the para aramid structure to form aramid III with a unique molecular structure. Aramid III has properties such as low density, ultra-high strength, ultra-high modulus, high temperature resistance, high impact resistance, wear resistance, and high transparency, and has broad application prospects. The high crystallinity, smooth surface, and strong chemical inertness of aramid lead to low bonding strength between fibers and resin in aramid reinforced composite materials (AFRP). In order to improve the interfacial bonding strength between fibers and resins, surface modification of fibers can be carried out by introducing chemically active groups or changing the microstructure of the surface to enhance the interaction between fibers and resins. The commonly used methods for fiber surface modification include physical and chemical methods. Chemical modification includes methods such as chemical oxidation, chemical grafting, and coupling agent modification.

However, most of the above surface modification methods use chemical reagents, which are complex to operate and pollute the environment. Plasma treatment can not only etch the surface of fibers to improve their roughness, but also break chemical bonds for grafting and polymerization. The method is simple, environmentally friendly, and efficient. The effect of cleaning and etching with argon gas is better and more efficient. This article uses argon plasma to treat domestic aramid III fibers, studies the effect of treatment on the fiber and the interfacial properties between the fiber and epoxy resin, and reveals its mechanism.

The Effect of Plasma Treatment on the Surface Chemical Composition of Aromatic III Fibers

Argon plasma treatment can destroy weak chemical bonds on the fiber surface and generate new reaction sites, while grafting introduces some active groups that alter the chemical properties of the fiber surface. Figure 1 shows the XPS full scan image and elemental content of Fang III. It can be seen that the surface of the aromatic III precursor is composed of three elements: C, N, and O, accounting for 81.24%, 5.97%, and 12.79%, respectively. The argon plasma treatment for 5 minutes resulted in a significant change in the proportion, with the proportion of N and 0 on the fiber surface increasing to 9.71% and 17.41%, respectively, and the proportion at ℃ decreasing to 72.88%. When the processing time was extended to 15 minutes and 20 minutes, the N content increased to 12.34% and 12.22%, respectively, and the 0 content was 18.28% and 18.32%, respectively. It can be inferred that during the argon plasma treatment, the chemical bonds on the fiber surface were broken and some active groups containing N and O were introduced. The introduced O-containing groups are more easily saturated than N-containing groups.

Figure 1 XPS full spectrum scanning results of aromatic III after argon plasma treatment

Figure 2 C1s peak spectrum of aromatic III after argon plasma treatment

Figure 2 shows the chemical changes on the surface of aromatic III, as well as the fine spectral peaks of C, N, O and C1s. As shown in Figure 2, the C1s of the aromatic III precursor have three peaks: - C-C - (284.5eV), - C-N - (285.5eV), and - C=O (287.8eV), with contents of 83.4%, 13.3%, and 3.3%, respectively; After being treated with argon plasma for 5 minutes, two new functional groups - C-O - (286.5eV) and - O-C=O (289.1eV) appeared on the fiber surface [34]. The content of - C-C - decreased to 68.07%, while the content of - C-N -, - C-O -, - C=O, and - COO - increased to 68.07%, 14.25%, 4.38%, 7.49%, and 5.81%, respectively.

With the extension of processing time, the content of C-C continued to decrease, decreasing to 61.38% and 55.33% at 15 and 20 minutes of treatment, respectively. At 20 minutes of plasma treatment, the content of - C-N -, - C-O -, - C=O, and - COO - increased to 17.87%, 7.77%, 11.35%, and 7.68%, respectively. The decrease in C-C content and the generation of C-O and COO indicate that argon plasma may damage the C-C on the fiber surface. The free radicals in the grafting plasma form new active groups, reducing the stability and surface inertness of the fiber and increasing its chemical activity. This series of changes helps to improve the wettability of the fiber surface and enhance the reactivity between the fiber and the matrix, resulting in an increase in interfacial strength.

Figure 3 N1s peak spectrum of aromatic III surface after argon plasma treatment

Figure 3 shows the peak splitting results for N1s. It can be seen that there are only two potential peaks in the N1s of the aromatic III precursor, which are attributed to the N-heterocyclic ring (- N=C/N-C) at 389.3 eV and the - NH-CO (400 eV) connected to the benzene ring. The destruction of NH-CO by plasma treatment resulted in a decrease in its overall concentration. After 5 minutes of plasma treatment, the concentration decreased from 84.1% to 73.49%, and a new functional group - NH2 was generated at 401.4 eV. However, the stable structure of the N-heterocyclic ring was beneficial, and the proportion of - N=C/N-C did not change significantly. This indicates that the stability of only - NH-CO among the N functional groups on the fiber surface after plasma treatment is reduced and - NH2 is generated. At the same time, on the surface of fibers treated with plasma for 20-30 minutes, the content of NH-CO increases to a certain extent with the extension of treatment time, which may be due to the recombination of partially free ⋅ NH2 with the cleaved ⋅ C=O.

Figure 4 O1s peak spectrum of aromatic III surface after argon plasma treatment

Figure 4 shows the peak separation results of O1s. It can be seen that the O1s of untreated aromatic III only has one potential peak of O=CNH, and its content decreases after plasma treatment. After 5 minutes of plasma treatment, the content decreased from 100% to 60.20% and two oxygen-containing groups, O=C-O and HO-C, were added. This may be due to the formation of a new group O=C-O by breaking the bond between the generated ⋅ OH and amide, or by combining with the benzene ring to form HO-C. The newly generated O=C-O and HO-C are beneficial for improving the surface wettability and chemical bonding ability of fibers. The reference positions for the binding energies of three oxygen-containing functional groups are 531.6 eV (O=CNH), 532.8 eV (O=C-O), and 533.7 eV (HO-C), respectively.

The Effect of Plasma Treatment on Surface Wettability

The wetting performance of fiber surface is closely related to surface energy. The higher the surface energy, the stronger the wetting performance of fiber surface, and the easier it is for resin to infiltrate the fiber to form a stable and firm interface. As shown in Figure 5b, the wettability of aromatic III treated with gas plasma for 5 minutes is greatly improved, with a surface energy of 68.52 mJ/m2, which is 48.50% higher than the 46.14 mJ/m2 of the original filament.

Figure 5: Dynamic Contact Angle and Surface Energy of Aromatic III after Argon Plasma Treatment

During this process, the polarity component plays a dominant role. As shown in Figure 5a, the contact angle of aromatic III with water sharply decreases from 71.93 ° of the original filament to 26.57 °, but the change in contact angle with diiodomethane is not significant. The contact angle with water is influenced by various factors. The content of polar functional groups and surface morphology both affect the contact angle. Argon plasma treatment not only increases the content of C-O, C=O, COOH, and - NH2 hydrophilic groups on the fiber surface, but also etches the fiber surface to increase the wetting area of the liquid on the fiber, improving the wettability of the aromatic III surface from both physical and chemical aspects. However, with the increase of plasma treatment time, the change trend of water contact angle and surface energy of aromatic III is consistent with the change trend of surface roughness. This indicates that physical etching with argon plasma treatment is more important for improving the wetting performance of aromatic III surface compared to chemical reactions.

In summary, treating aromatic III fibers with argon plasma for an appropriate treatment time can significantly increase the content of polar groups, roughness, and wetting properties on the fiber surface.