Briefly Describe the Surface Treatment Technology of Carbon Fiber

 

Carbon fiber has excellent performance, but it is difficult to use alone. It must be used as a reinforcement for advanced composite materials. However, carbon fiber is prepared by high-temperature carbonization. During the carbonization process, non-carbon elements gradually escape and inert carbon elements are enriched, resulting in a high chemical inertness on the surface of the carbon fiber. Therefore, the surface of the carbon fiber must be modified.

 

Classification And Effectiveness Of Carbon Fiber Surface Treatment

 

Carbon fiber surface treatment is mainly divided into two categories, oxidation treatment and non-oxidation treatment. Oxidation treatment can be divided into liquid phase oxidation, electrochemical oxidation, ozone oxidation, etc., while non-oxidation treatment mainly includes coating treatment, thermal deposition, etc. Among the many modification processes, electrochemical oxidation treatment is the most gentle and easy to control, and it is also the only modification method that can be achieved online.

 

After surface modification, the microscopic physical and chemical structure of carbon fiber will change significantly, and it is conducive to the improvement of the interface bonding strength of carbon fiber reinforced composite materials. On the one hand, the surface roughness of carbon fiber increases, which will increase the physical intercalation between the matrix and the carbon fiber; on the other hand, after surface modification, carbon fiber will produce a large number of oxygen-containing active groups such as -OH and -COOH, which will help to improve the chemical bonding between carbon fiber and the matrix when preparing composite materials. After surface treatment, the interface bonding strength of carbon fiber reinforced composite materials will be greatly improved.

Several Surface Treatment Methods for Carbon Fiber

 

1. Liquid Phase Oxidation Method

 

The surface of carbon fiber is oxidized with an oxidizing solution. The main factors affecting the oxidation effect include: solution concentration, oxidation time, treatment temperature, etc.

Figure 1 shows the liquid phase oxidation of the carbon fiber surface using nitric acid at 110°C, with a treatment time of 30-180 minutes. It can be seen from the figure that with the increase of treatment time, the roughness of the fiber surface increases.

 

However, due to the long oxidation time, the fiber surface is severely etched, resulting in a significant decrease in the mechanical properties of the fiber. After three hours of oxidation, the mechanical properties of the fiber decreased by nearly 40%.

 

 

Figure 1 Liquid Phase Oxidation Surface Treatment of Carbon Fiber, (a) Untreated, (b-f) Oxidation Time is 30, 60, 90, 120, 180 Min Respectively

 

2. Plasma Treatment

Plasma treatment is carried out in a plasma generator. Commonly used plasmas include oxidative atmospheres of air, oxygen, and ozone. In situations where there are special requirements for the surface active groups of carbon fibers, gases such as nitrogen can also be used to increase the nitrogen-containing functional groups on the fiber surface.

 

Plasma treatment has high requirements for the device, and factors such as concentration are difficult to control during the treatment process. Therefore, it is only suitable for laboratory research. Figure 2 shows the surface structure of carbon fibers before and after oxygen plasma treatment.

 

Figure 2 Carbon Fiber Plasma Treatment

 

3. Irradiation Treatment

In this modification method, the carbon fiber is exposed to gamma rays or laser rays to increase the surface roughness of the carbon fiber. After irradiation treatment, the surface roughness of the carbon fiber is greatly increased, which is conducive to the mechanical interlocking of the resin matrix with the fiber surface.

 

The carbon fiber fabric is subjected to modification treatment under different doses of radiation sources, as shown in Figure 3. The results show that as the dose increases from 0-300kGy, the roughness of the fiber surface increases. Raman spectroscopy confirms that the degree of disorder of the fiber surface ID/IG increases after modification. After modification, the interlaminar shear strength of the composite material is increased by up to 60%

 

Figure 3 Irradiation Modification of Carbon Fiber Surface