Continuous carbon fiber reinforced PA66 composites have shown broad application prospects in the fields of automobile lightweight and aviation due to their high specific strength, chemical corrosion resistance and recyclability. However, there are significant interfacial bonding defects between carbon fiber and PA66 resin matrix, which is mainly due to the incompatibility between the chemical inertness of carbon fiber surface and the polar groups of PA66, which seriously affects the mechanical properties of the composites. Although traditional interface modification technologies ( such as liquid phase oxidation and coupling agent modification ) can improve the interface bonding strength, the process time is long. In contrast, plasma surface treatment technology has gradually become the technical choice of fiber online surface treatment due to its advantages of simple operation, green pollution-free and low cost.

The on-line plasma surface treatment device mainly produces air through the air compressor, and then controls the gas pressure through the air pressure valve and enters the plasma generator. After the gas is ionized by the generator, the plasma is sprayed by the rotary jet instrument to improve the wetting performance of the fiber surface and increase the roughness of the surface etching. The online plasma treatment device is shown in Fig.1-1 :

Fig. 1-1 On-line plasma treatment of carbon fiber

In the process of plasma surface treatment, the surface of the fiber reinforcement is mainly affected by the following dual mechanisms : on the one hand, the surface of the fiber is etched and modified to increase the surface roughness of the fiber. The specific surface area of the fiber increases, which increases the contact area between the resin and the fiber to a certain extent and forms an interlocking structure to increase the interfacial bonding force. On the other hand, different gas plasma treatments can activate the elements on the fiber surface.

Study on the mechanism of on-line plasma treatment of CF interface chemical structure

Table 1-1 Changes of element content on carbon fiber surface before and after plasma treatment

From Table 1-1, it can be seen that the content of C element in the fiber treated by plasma decreased by 2.07 %, the content of O element increased by 2.12 %, and the content of N element remained basically unchanged. The O / C element ratio increased by 12 %. This indicates that the oxygen-containing elements and oxygen-containing functional groups on the surface of the carbon fiber treated by the plasma increase, and the surface activity of the fiber is activated.

The proportion of different carbon-containing functional groups was determined by the peak area of the C1 s orbital peak splitting spectra of the fibers before and after treatment, as shown in Figure 2-1 :

Fig.2-1 Fitting curves of C1 s peaks on CF surface before and after plasma treatment

Among the carbon-containing functional groups on the surface of carbon fibers treated by plasma, the proportion of C-C bonds decreased and the proportion of O-containing functional groups increased. Among them, the increase of C-O content can effectively improve the surface wettability of the fiber and promote the infiltration of the resin matrix to the fiber. At the same time, the increase of C = O functional group content can form a strong interfacial interaction with PA66 and other matrices. The chemical structure of CF treated by air plasma is mainly to enhance the chemical bonding effect by introducing more oxygen-containing functional groups to form covalent bonds with the polar groups of the resin matrix.

The fitting diagram of O1 s peak on CF surface before and after plasma treatment is shown in Figure 3-1 :

Fig.3-1 Fitting curves of O1 s peaks on CF surface before and after plasma treatment

After surface treatment, the proportion of C = O decreased and the proportion of C-O increased. This is because the content of C-O bond-related functional groups on the surface of the sample increased by plasma treatment. The treatment process promoted the oxidation reaction, introduced more C-O-containing structures ( such as hydroxyl, ether bond, etc. ), changed the chemical activity of the fiber surface, and increased the proportion of oxygen-containing functional groups on the fiber surface.

As an environmentally friendly dry process, plasma surface treatment technology is based on the principle of non-equilibrium plasma activation and does not rely on a large amount of water and chemical solvents. Therefore, it has the characteristics of environmental friendliness, simple operation and low cost. This technology can be widely used in the surface modification of carbon fiber materials, and there is no specific restriction on the treatment object, showing a strong versatility.