Carbon fiber Pre-Oxidation and Carbonation Process
Carbon fiber precursor, Polyacrylonitrile fiber (PAN) coming from the unwinding area is relatively dry. After the atomized spray moisturizing, it is not easy to stick together, so that it can enter the Pre-oxidation Furnace for a better series of reactions.
The pre-oxidation process is a key step in the production of carbon fiber. After the pre-oxidation process, the precursor is transformed from a linear molecular chain to a heat-resistant ladder structure, which plays the role of fixing oxygen and carbon for the subsequent carbonization process. In this process, a series of reactions such as cyclization, oxidation and dehydrogenation occur. The original straight chain structure dominated by σ bonds forms a large number of delocalized π electrons, forming a color-producing conjugated structure. The precursor gradually changes from pure white to darker: white → light amber → beige → light brown → brown → black. In this process, PAN undergoes a chemical reaction to remove a large number of small molecules and undergo structural changes. A certain stretching force needs to be applied to ensure that the structure of the fiber does not undergo deorientation and to ensure the strength of the pre-oxidized fiber. The temperature during the pre-oxidation process is between 200℃ and 300℃. At around 240℃, the oxygen content rises rapidly and a chemical reaction occurs. During this process, the temperature in the furnace is kept uniform, and circulating air takes away small molecules and other impurities and reaction heat generated in the reaction, ensuring that pre-oxidation can proceed continuously. The pre-oxidation process takes a long time, 80 to 100 minutes, which restricts the production efficiency of carbon fiber. The workshop uses three pre-oxidation furnaces to pre-oxidize the fiber at the same time, so that the fiber has enough pre-oxidation reaction time, and can be continuously fed for assembly line production.
The PAN raw fiber is converted into pre-oxidized fiber with 8% to 10% oxygen by pre-oxidation, and then enters the Carbonization Furnace for carbonization. The temperature of the low-temperature carbonization furnace is between 300℃ and 800℃, which is divided into several temperature intervals. The pre-oxidized fiber is gradually carbonized under the condition of air isolation to form a primary turbostratic graphite structure. During the pre-oxidation, the pre-oxidized fiber undergoes pyrolysis and polycondensation reactions, which will produce a large amount of waste gas and tar, which should be discharged in time through the exhaust port to ensure stable production. The high-temperature carbonization temperature is between 1000℃ and 1600℃, generally around 1400℃. During this process, the pre-oxidized fiber undergoes further reaction to form a turbostratic graphite structure and release some small molecules. During the carbonization process, a change in molecular structure occurs, and a certain stretching force should be given to ensure the orientation of the structure.
In order to better use carbon fiber in composite material production, the surface of carbon fiber needs to be treated to form a better contact surface. Using the anodic electrode oxidation method, the surface treatment is carried out by pulse power supply, so that the carbon fiber surface is etched and oxygen-containing functional groups are produced. During the surface treatment, a voltage of about 10V is passed to form a current of about 25A. Ammonium bicarbonate neutral electrolyte is used for surface treatment.
The carbon filaments are cleaned by the immersion method during the water washing process to remove impurities such as electrolyte on the surface of the carbon filaments in preparation for the subsequent sizing process. The water temperature is set at about 50℃ during the water washing, and then a drying process of about 100℃ is performed.
Carbon fiber is a brittle material, and it is easy to have undesirable phenomena such as fuzzing in the subsequent deep processing process. The sizing process can well improve the surface properties of the carbon fiber. After sizing, a layer of film is formed on the surface of the carbon filament, which makes the carbon filament have good bundling, improves wear resistance, reduces fuzzing in deep processing, and reduces the water absorption of the carbon fiber, making it easy to open and expand the fiber. It has better stability, can improve the properties of the contact surface in the composite material, and increase the strength of the material. The sizing process uses the impregnation method to make a layer of sizing agent adhere to each carbon fiber monofilament, and after drying, a thin cortex is formed on the carbon fiber to complete the sizing. During the process, the sizing agent attached to the surface of the carbon fiber should be uniform, without dead areas and enriched areas, to ensure uniform sizing. After sizing, after drying (about 180℃), the finished carbon fiber can be obtained by collecting the filaments.
Key words: carbon fiber, Polyacrylonitrile fiber, PAN, pre-oxidation, carbonation