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What is the purpose of pre-oxidation in carbon fiber production?

2026-01-10 16:36:49

27421

The pre-oxidation process is a core and indispensable step in carbon fiber production (especially for the mainstream PAN-based carbon fiber), and its core purpose is to transform the thermoplastic precursor fiber into a thermoset pre-oxidized fiber that does not melt or stick during subsequent high-temperature carbonization, laying the foundation for forming high-performance carbon fiber with a continuous, oriented structure.

Specifically, the purpose of pre-oxidation can be divided into the following four key aspects, tailored for industrial production scenarios:

1. Eliminate thermoplasticity and prevent melting during carbonization

PAN precursor fiber is a linear thermoplastic polymer—if it is directly sent to the carbonization furnace (800–1500°C), it will melt, stick together, and even decompose completely, failing to maintain the filamentary shape required for carbon fiber.

During pre-oxidation (heating to 200–300°C in air atmosphere), the PAN molecular chains undergo cyclization, dehydrogenation, and oxidation cross-linking reactions, forming a rigid three-dimensional network structure. This structure completely eliminates the thermoplasticity of the precursor, making the fiber non-melting and non-sticky at high temperatures. It can maintain a continuous filament shape during subsequent carbonization and graphitization, which is the prerequisite for producing carbon fiber.

2. Remove partial non-carbon elements and improve carbon yield

The PAN precursor contains a large number of non-carbon elements (hydrogen, nitrogen, oxygen, etc.). Pre-oxidation initiates the preliminary removal of these elements through pyrolysis reactions:

Dehydrogenation reaction: Removes hydrogen in the form of H₂;
Deamination reaction: Removes nitrogen-containing small molecules such as NH₃;
Oxidation reaction: Converts partial carbon into CO or CO₂.

This step reduces the content of non-carbon elements in the fiber in advance, avoiding the violent escape of a large number of small molecules during high-temperature carbonization (which would form bubbles and pores inside the fiber). Ultimately, it improves the carbon yield of the subsequent process—the carbon yield of PAN-based pre-oxidized fiber after carbonization can reach 50–60%, while the direct carbonization of untreated precursor would have a carbon yield of less than 30%.

3. Construct a preliminary oriented structure and lay the foundation for high modulus

During pre-oxidation, appropriate tension (0.3–0.8 cN/dtex) is applied to the fiber bundle. This tension can align the cross-linked molecular chains along the fiber axis, forming a preliminary oriented structure.

In the subsequent carbonization and graphitization stages, this oriented structure will be further optimized—carbon atoms will arrange into ordered graphite microcrystals along the axial direction of the fiber. This is the core reason why carbon fiber has ultra-high elastic modulus (200–800 GPa). Without the oriented structure formed during pre-oxidation, the final carbon fiber will have a loose and disordered structure, and its modulus will be drastically reduced.

4. Improve the thermal stability of the fiber and adapt to extreme high-temperature processes

The three-dimensional cross-linked structure formed by pre-oxidation has excellent thermal stability—it can withstand the high temperature of 1500°C in the carbonization furnace without structural collapse.

In contrast, untreated PAN precursor will decompose rapidly at temperatures above 300°C. Therefore, pre-oxidation is equivalent to "stabilizing" the fiber structure, enabling it to adapt to the extreme high-temperature environment of subsequent carbonization and graphitization, and ensuring the smooth progress of the entire carbon fiber production process.

Key Conclusion

Pre-oxidation is the "structural transformation link" in carbon fiber production—it solves the core problem of "precursor melting during high-temperature carbonization", and at the same time optimizes the fiber structure and improves carbon yield. The quality of pre-oxidation (degree of oxidation, structural uniformity) directly determines the mechanical properties (strength, modulus) of the final carbon fiber.

Carbon Fiber,Pre-oxidized Fiber,Carbon Fiber Prepreg

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