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What are the key steps in the preparation process of pre-oxidized fiber?

2026-01-10 14:33:41

20646

Pre-oxidized fiber (also called stabilized fiber) is a critical intermediate product in carbon fiber production—its quality directly determines the mechanical properties of the final carbon fiber. The preparation process is mainly aimed at precursor filaments (PAN-based, pitch-based, or rayon-based), with PAN-based precursors being the most common in industrial production. Below are the key steps and technical control points for the industrial preparation of pre-oxidized fiber:

1. Pre-Treatment of Precursor Filaments

Before pre-oxidation, the precursor filaments need to be processed to ensure uniform tension and prevent breakage during high-temperature treatment:

Unwinding and Guiding: Precursor filament rolls are unwound at a constant speed, and the filaments are guided through a series of rollers to maintain consistent tension (typically 0.1–0.3 cN/dtex for PAN filaments). Uneven tension will cause fiber diameter variation or breakage in subsequent steps.
Optional Sizing Removal: For some precursor filaments with surface sizing agents (used to protect filaments during storage), a mild washing or heating step is added to remove the sizing, avoiding its decomposition and contamination during high-temperature pre-oxidation.

2. Staged Pre-Oxidation Heating (Core Step)

This is the most critical step in preparing pre-oxidized fiber, and industrial production uses continuous multi-zone heating furnaces to achieve staged temperature control. Taking PAN-based precursors as an example, the process is divided into 3–4 temperature zones, with the temperature gradually increasing from low to high:

Zone 1 (Low-Temperature Stage: 200–220°C): The main reactions are water evaporation and initial cyclization of PAN molecular chains. The moisture in the filaments is removed to prevent bubble formation in subsequent high-temperature steps, and the nitrile groups (-CN) in PAN start to react to form stable cyclic structures. The heating time here is relatively short (1–2 minutes), and the atmosphere is air.
Zone 2 (Medium-Temperature Stage: 220–250°C): The core reaction is intensive cyclization and dehydrogenation. The cyclic structures of PAN molecular chains are further formed, and hydrogen atoms are removed to form conjugated double bonds. This step is key to improving the thermal stability of the filaments—without sufficient dehydrogenation, the filaments will melt and stick together during carbonization. The atmosphere remains air, and the heating time is extended to 2–3 minutes.
Zone 3 (High-Temperature Stage: 250–280°C): The reactions include oxidation cross-linking and stabilization of molecular chains. Oxygen in the air participates in the reaction to form oxygen-containing groups (e.g., -COOH, -OH) on the fiber surface, which enhance the cross-linking density of molecular chains. At this stage, the filaments are transformed from thermoplastic to thermoset, and they no longer melt when heated to higher temperatures. The heating time is 3–5 minutes, and the temperature must be strictly controlled to avoid over-oxidation (which causes fiber brittleness).

Key Control Parameters:

Heating rate: 1–3°C/min (too fast causes uneven reaction; too slow reduces production efficiency).
Tension control: Maintain a small amount of stretching (2–5% elongation) during heating to align molecular chains, which improves the modulus of the final carbon fiber.
Atmosphere: Air is used for PAN-based precursors (provides oxygen for oxidation cross-linking); inert atmosphere (nitrogen) is used for some pitch-based precursors to avoid excessive oxidation.

3. Cooling and Conditioning

After the multi-zone pre-oxidation treatment, the high-temperature pre-oxidized fibers need to be cooled to room temperature to stabilize their structure:

Controlled Cooling: The fibers pass through a cooling zone with gradually decreasing temperature (from 280°C to room temperature) to avoid thermal shock, which can cause microcracks in the fibers.
Humidity Conditioning: The cooled pre-oxidized fibers are passed through a humidification zone (relative humidity 40–60%) to adjust their moisture content, preventing static electricity and improving the processability of subsequent carbonization steps.

4. Winding and Quality Inspection

Winding: The pre-oxidized fibers are wound into rolls at a constant speed, with the winding tension matching the pre-oxidation tension to prevent fiber deformation.
Online Quality Inspection: Industrial lines use sensors to detect key indicators of pre-oxidized fibers in real time:
- Linear density deviation: Control within ±3% to ensure uniform fiber diameter.
- Breaking strength: PAN-based pre-oxidized fibers typically have a breaking strength of 2.0–3.0 cN/dtex.
- Oxidation degree: Determined by measuring the fiber’s oxygen content (typically 10–15% for PAN-based pre-oxidized fibers) or density (1.3–1.4 g/cm³). Defective fibers are automatically cut and removed.

Core Technical Purpose of Pre-Oxidation

The entire process is designed to transform the thermoplastic precursor filaments into thermoset pre-oxidized fibers with stable chemical structures. This prevents the filaments from melting, sticking, or decomposing during the subsequent high-temperature carbonization (1000–1500°C) and graphitization (2000–3000°C) steps, ensuring the formation of high-strength, high-modulus carbon fibers.

Carbon Fiber,Pre-oxidized Fiber,Carbon Fiber Prepreg

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