The function of the pre-oxidation process is to stabilize the lignin fibers, effectively chemically "locking" their structure so they do not melt or fuse together during later high-temperature processing. Conducted in a tube furnace at 200–270°C in an air atmosphere, this step triggers specific chemical reactions—dehydrogenation, oxidation, and cross-linking—that increase the material's heat resistance and maintain its nanostructure.
Core Insight: Pre-oxidation is the bridge between a raw, meltable polymer and a heat-resistant carbon precursor. It transforms the lignin from a thermoplastic state (which melts under heat) into a thermoset ladder-like structure (which retains its shape), allowing the fiber to survive the extreme temperatures of graphitization.
The Mechanics of Stabilization
The Chemical Transformation
Inside the tube furnace, the lignin undergoes a complex series of reactions, primarily dehydrogenation, oxidation, and cross-linking.
Supplementary data indicates that aromatization also occurs during this phase.
These reactions strip away hydrogen and introduce oxygen bridges between molecules.
From Linear to Ladder Structure
Raw lignin molecules are typically linear or branched.
Pre-oxidation forces these molecules to connect into a highly cross-linked, ladder-like structure.
This structural rigidity is what gives the fiber its new-found thermal stability.
The Thermoplastic to Thermoset Shift
Initially, lignin is thermoplastic, meaning it softens and melts when heated.
Through pre-oxidation, the material becomes thermoset.
This means it will degrade rather than melt at high temperatures, which is essential for preserving the fiber's physical form.
The Critical Role of the Tube Furnace
Precise Atmosphere Control
Unlike the subsequent carbonization step which requires inert nitrogen, pre-oxidation requires an air atmosphere.
The oxygen in the air is a necessary reactant to facilitate the cross-linking process.
Temperature Uniformity
The process operates within a tight window of 200–270°C.
The tube furnace provides the precise thermal field uniformity required to ensure every inch of the fiber reacts at the same rate.
Uneven heating would lead to weak points or "un-stabilized" sections in the fiber.
Understanding the Trade-offs
The Risk of Overheating
While heat is necessary for the reaction, exceeding 270°C in an air environment can lead to combustion.
If the temperature rises too high before the structure is stabilized, the fibers may simply burn up rather than cross-link.
The Consequence of Under-processing
Conversely, if the temperature is too low or the dwell time too short, the core of the fiber may remain thermoplastic.
This leads to the melting or adhering of fibers during the subsequent high-temperature carbonization (up to 1800°C).
If fibers fuse together, the mechanical integrity of the final carbon fiber product is compromised.
Making the Right Choice for Your Goal
To ensure high-quality carbon fiber production, you must view pre-oxidation as the foundation of structural integrity.
- If your primary focus is Structural Integrity: Ensure your heating profile is slow and uniform to allow full oxygen diffusion into the fiber core, preventing a "skin-core" effect where only the outside is stabilized.
- If your primary focus is Process Efficiency: Optimize the upper limit of the 200–270°C range to accelerate cross-linking, but implement rigorous temperature controls to prevent oxidative damage or burning.
The quality of your final carbon fiber is determined not by the extreme heat of carbonization, but by the precision of this initial stabilization phase.
Summary Table:
| Feature | Details |
|---|---|
| Temperature Range | 200°C – 270°C |
| Atmosphere | Air (Oxygen required for cross-linking) |
| Chemical Processes | Dehydrogenation, Oxidation, Cross-linking, Aromatization |
| Material Transformation | Thermoplastic (meltable) to Thermoset (heat-resistant) |
| Core Function | Prevents fiber fusing/melting during high-temp carbonization |
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References
- Changyu Shen, Jun Li. Investigation on spinnability of low molecular weight alkaline lignin to fabricate biobased carbon fiber. DOI: 10.15251/djnb.2024.191.417
This article is also based on technical information from Kintek Furnace Knowledge Base .
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