Epoxy resin (EP) has excellent comprehensive properties such as heat resistance, mechanical properties, processability, electrical insulation, stability, etc., and is widely used in fields such as electronic devices, construction engineering, automotive parts, aerospace instruments and equipment. However, EP also has some problems such as high cross-linking density and susceptibility to cracking caused by internal stress, poor impact toughness, and poor flame retardancy. To improve the comprehensive performance of EP, it is usually compounded with organic or inorganic fillers such as glass fiber, carbon fiber, alumina, hollow glass beads (HGB), etc. Among them, HGB is often used for lightweight and high-strength modification of EP due to its characteristics of light weight, high strength, easy dispersion, excellent insulation and heat resistance. Usually, unmodified fillers have smooth surfaces and no active functional groups, and the interaction force between them and the polymer matrix is weak, making it difficult to effectively transfer and diffuse the stress they carry, and thus unable to achieve high comprehensive performance.

To improve the interfacial adhesion between HGB and polymer matrix, some researchers have used coupling agents, polydopamine, water-based sizing agents, etc. to modify the surface of HGB. The above methods mainly use chemical reagents to introduce active groups on the surface of HGB, which is inefficient and has a certain negative impact on the environment. Physical surface modification methods such as plasma, high-energy radiation, ultrasound, etc. can also introduce active groups on the surface of HGB, avoiding the use of chemical reagents. Among them, plasma surface modification can transform the inert functional groups on the surface of the filler into active functional groups, thereby increasing the molecular interaction force between the filler and the matrix; Meanwhile, plasma surface treatment has a similar etching effect and can enhance mechanical interlocking.

Analysis of the influence of plasma treatment on the microstructure and surface functional groups of HGB

From Figure 1, it can be seen that after plasma surface treatment, the smoothness of HGB-0 surface decreases, the roughness increases, and burn like marks appear. This is because plasma treatment causes etching on the surface of HGB. As shown in Figure 2, after plasma surface treatment, HGB exhibits distinct new absorption peaks at 2918cm-1 and 2849cm-1, which are characteristic peaks of the C-H bond in the aldehyde group. This indicates that the vacuum plasma surface treatment successfully introduced active groups on the surface of HGB, which is beneficial for enhancing the interaction between HGB and EP.

Figure 1 SEM of HGB before and after modification

Figure 2 FTIR spectra of HGB before and after modification

Surface wettability analysis of HGB

The contact angle of water droplets falling on the surface of HGB is shown in Figure 3, and the time required for complete spreading (the time when the contact angle drops to 0 °) is shown in Table 1. According to Table 1, the contact angle of unmodified HGB during water droplet falling (0s) is 56.5 °, and it fully spreads after 3.2 seconds. After 1 hour of plasma surface treatment, the contact angle of HGB at 0 seconds and the complete water spreading time decreased to 47.3 ° and 2.4 seconds, respectively, indicating an improvement in wettability. This is because plasma surface treatment introduces active groups to the surface of HGB.

Figure 3 Contact angle photos of HGB before and after modification

Based on the above analysis, the active groups generated on the surface of HGB through plasma treatment can significantly improve its surface wettability, enhance the interaction force between HGB and EP interface, and effectively improve the mechanical properties of composite materials.