How to improve the aging resistance of pre-oxidized fiber?

Improving the aging resistance of pre-oxidized fiber is critical for extending its storage life, maintaining processability during carbon fiber production, and ensuring the quality of the final carbon fiber. Aging of pre-oxidized fiber mainly manifests as structural brittleness, surface oxidation, and reduced carbon yield caused by long-term exposure to oxygen, moisture, light, and temperature fluctuations. Below are targeted technical measures tailored for industrial production scenarios:

1. Optimize Storage Conditions to Isolate Aging Triggers

The most direct way to prevent aging is to block the contact between pre-oxidized fiber and external aging factors, which is the basis of aging resistance improvement.

• Sealed vacuum packaging with anti-oxidation additives

  • Package pre-oxidized fiber rolls in aluminum foil vacuum bags with oxygen absorbents (e.g., iron-based oxygen scavengers) and desiccants (e.g., silica gel, molecular sieves). This reduces the oxygen concentration inside the package to below 0.1% and moisture content to <0.5%, preventing oxidative hydrolysis of the fiber’s surface oxygen-containing groups.

  • Add a small amount of antioxidant masterbatch (e.g., hindered phenols, phosphites) to the packaging. These additives can capture free radicals generated by slow oxidation, inhibiting the chain reaction of fiber structure degradation.

• Controlled low-temperature and dark storage

  • Store packaged pre-oxidized fiber at 5–10°C (avoid freezing, which may cause fiber embrittlement). Low temperatures slow down the kinetic rate of oxidation reactions—tests show that storage at 5°C reduces the aging rate by 60% compared to room temperature storage.

  • Keep the warehouse dark (avoid direct sunlight or UV exposure). UV rays can break the conjugated double bonds in the pre-oxidized fiber’s cross-linked structure, leading to surface cracking and brittleness. Cover the fiber rolls with opaque plastic films if necessary.

• Avoid mechanical stress during storage

  • Place pre-oxidized fiber rolls flat on pallets with no more than 3 layers of stacking. Excessive pressure will cause fiber deformation and internal microcracks, which become channels for oxygen and moisture to accelerate aging.

2. Modify the Fiber Surface to Enhance Barrier Properties

Surface modification forms a protective layer on pre-oxidized fiber, isolating it from external corrosive factors while maintaining its processability for subsequent carbonization.

• Thin-layer coating of inert resin

  • Apply a 0.5–1 μm thick water-based epoxy resin or silicone resin coating on the fiber surface via dip coating or spray coating. The resin forms a dense, breathable film that blocks oxygen and moisture but does not affect the release of small molecules during carbonization.

  • Key requirement: Choose low-molecular-weight resins that can decompose completely at 300–400°C without leaving carbon residues, avoiding impact on the final carbon fiber’s purity.

• Surface grafting of anti-aging functional groups

  • Use plasma treatment to activate the pre-oxidized fiber surface, then graft silane coupling agents (e.g., KH-550) with amino groups. The grafted groups form a hydrophobic layer on the fiber surface, reducing moisture absorption by 40–50% and inhibiting surface oxidation.

  • This method does not introduce additional impurities and can also improve the interface bonding between pre-oxidized fiber and resin matrices (if used directly for composite materials).

3. Optimize Pre-oxidation Process to Improve Intrinsic Structural Stability

The aging resistance of pre-oxidized fiber is fundamentally determined by its internal cross-linked structure. Optimizing the pre-oxidation process to form a more stable molecular framework can enhance its inherent aging resistance.

• Precise control of pre-oxidation degree (moderate oxidation)

  • Avoid under-oxidation or over-oxidation: Under-oxidized fiber has insufficient cross-linking and is prone to structural degradation; over-oxidized fiber has excessive surface oxygen-containing groups and is more susceptible to hydrolysis.

  • Target parameters: Control the fiber density at 1.3–1.4 g/cm³ and oxygen content at 10–15% (PAN-based pre-oxidized fiber). This forms a balanced three-dimensional cross-linked structure with high stability.

• Staged tension adjustment during pre-oxidation

  • Apply gradient tension (0.3–0.8 cN/dtex) in different temperature zones of pre-oxidation. Proper tension aligns the molecular chains along the fiber axis, increasing the density of the cross-linked structure and reducing the number of structural defects (e.g., microvoids) that accelerate aging.

• Add anti-aging additives during precursor spinning

  • Incorporate a small amount of nanoparticle additives (e.g., TiO₂, SiO₂) into the PAN precursor solution before spinning. These nanoparticles are uniformly dispersed in the fiber and act as "reinforcement points" in the pre-oxidized fiber structure, inhibiting the expansion of microcracks during aging.

4. Strengthen Post-production Treatment and Process Management

• Surface passivation treatment

  • After pre-oxidation, treat the fiber with low-concentration ammonia water vapor (5–10%) at 60–80°C for 30 minutes. This neutralizes the acidic oxygen-containing groups (e.g., -COOH) on the fiber surface, reducing their reactivity and slowing down hydrolysis aging.

• Shorten storage time and optimize production scheduling

  • Minimize the storage period of pre-oxidized fiber—ideally, process it into carbon fiber within 1–2 months of production. If long-term storage is required, conduct a quality inspection (e.g., tensile strength test, carbon yield test) before use to eliminate aged fibers.

  • Adopt a just-in-time (JIT) production model to connect pre-oxidation and carbonization processes directly, reducing the exposure time of pre-oxidized fiber to the external environment.

Key Industrial Effect Verification Indicators

After implementing the above measures, the aging resistance of pre-oxidized fiber can be evaluated by the following indicators:

1. Tensile strength retention rate: >90% after 6 months of storage (vs. <70% for untreated fiber).

2. Carbon yield stability: Fluctuation range <3% after storage (vs. >8% for untreated fiber).

3. Surface defect rate: <2% of fiber length has cracks or oxidation spots (vs. >15% for untreated fiber).