Author: [Bruce Zhou]
Affiliation: [Jota Machinery Composites Material Prepreg Solution]
Corresponding Author: [jotamachinery@gmail.com]
Published : December 05 , 2025

Abstract

Polyacrylonitrile (PAN) remains the dominant precursor for structural carbon fibers, yet the cost and energy intensity of oxidative stabilization continue to restrict broader adoption in automotive, wind energy, and infrastructure applications. This paper consolidates recent advances on the interplay between PAN precursor design (molecular weight, polydispersity, tacticity, comonomer chemistry) and both conventional and alternative stabilization routes, with a focus on irradiation-assisted processes. Evidence from the literature indicates that high-molecular-weight (Mw > 200,000 g·mol⁻¹) PAN with narrow polydispersity and low levels of acidic comonomers (≈1–2 mol% itaconic acid or related monomers) provides favorable rheology, controlled cyclization kinetics, and improved final carbon fiber (CF) properties. At the same time, studies show that textile-grade PAN with higher comonomer content can still achieve automotive-grade CF performance when processing is carefully optimized.

Conventional stabilization in air requires multi-hour heat treatments and accounts for up to 75–80% of total processing time. In contrast, alternative strategies—microwave heating, plasma, ultraviolet irradiation, γ-rays, and electron-beam irradiation (EBI)—have demonstrated substantial reductions in stabilization time (from several hours to ≈10–40 minutes) and energy consumption, while attaining tensile strengths up to ≈8 GPa and moduli up to ≈655 GPa in specific microwave-assisted carbonization routes. The analysis suggests that “incomplete” stabilization (≈41–60% conversion; oxygen content ≈8–21 wt%; density ≈1.34–1.40 g·cm⁻³) can be sufficient, and often advantageous, for high tensile strength by limiting over-oxidation and defect formation; more extensive stabilization tends to favor higher modulus.

The review also highlights that the effects of tacticity, particularly isotactic enrichment, are more pronounced in reaction kinetics than in the final ladder polymer structure. Emerging eco-friendly process concepts—such as combining irradiation with melt-spun or textile-grade PAN and integrating low-temperature, short-time stabilization with advanced carbonization—offer promising pathways to cut precursor cost and specific energy demand without major performance penalties. Remaining challenges include scaling irradiation-based treatments to industrial tow counts, controlling radial heterogeneity during rapid stabilization, and establishing robust process windows for low-cost precursor grades.

Keywords

polyacrylonitrile; carbon fiber; oxidative stabilization; electron beam irradiation; microwave plasma; tacticity; comonomer; textile-grade PAN; high molecular weight PAN; sustainable carbon fiber processing

1. Introduction

Carbon fibers (CFs) combine high specific stiffness and strength with chemical resistance and fatigue durability, which has led to their widespread use in aerospace, sporting goods and, increasingly, in automotive and wind energy structures. Despite significant growth in global capacity, PAN-based CF remains relatively expensive, with precursor production and stabilization together accounting for the majority of cost. Market analyses typically assign 50–60% of CF manufacturing cost to the PAN precursor and stabilization step, while carbonization and surface treatment contribute the remainder.

From a processing standpoint, PAN-based CF production can be divided into four linked stages: (i) synthesis of the PAN copolymer, (ii) spinning and drawing of precursor fibers, (iii) stabilization (thermal oxidation and cyclization) to form an infusible ladder polymer, and (iv) carbonization/graphitization to generate a turbostratic or near-graphitic carbon structure. Among these, stabilization is the most time-consuming and energy-intensive, typically requiring 1–2 h for small tows and up to 5–14 h in large-tow industrial processes.

In parallel, there is sustained interest in making CF accessible beyond aerospace grade requirements. For many automotive and infrastructure applications, target properties around 1.7–2.0 GPa tensile strength and 170–200 GPa modulus are sufficient. This opens the door to alternative precursor grades, such as textile PAN or melt-spun PAN with higher comonomer content, provided that stabilization and carbonization are tailored accordingly.

The present review focuses on the link between PAN microstructure and emerging stabilization technologies, especially irradiation-assisted routes that aim to compress processing times from hours to tens of minutes. The discussion is structured around four themes:

1. PAN precursor design: molecular weight, polydispersity, tacticity and comonomer chemistry.
2. Structural transformations and process windows during conventional stabilization.
3. Alternative stabilization strategies (microwave, plasma, UV, γ, EBI) and their impact on CF performance.
4. Implications for cost, sustainability, and the use of lower-cost precursor grades.

2. PAN Precursor Design: Molecular Weight, Polydispersity and Comonomers

2.1 Molecular weight and polydispersity

For PAN-based CF, high molecular weight (Mw) and low polydispersity index (PDI) are generally favorable. Higher Mw increases chain entanglement and allows precursor drawing to higher orientation without premature breakage; narrow PDI ensures uniform rheology and spinning behavior.

Studies on solution-spun and gel-spun PAN show that Mw values greater than 200,000 g·mol⁻¹, and in some cases up to 1.7×10⁶ g·mol⁻¹, can be achieved via controlled radical polymerization (e.g., RAFT, ATRP) or template-assisted routes. When spinning and drawing are properly optimized, ultrahigh-MW PAN precursors can be converted into CF with tensile strengths exceeding 4 GPa and moduli above 350 GPa. A key mechanism is the possibility to form thinner filaments—around 4–6 μm diameter—reducing critical flaw size in line with Griffith’s fracture considerations.

At the same time, excessively broad distributions (PDI > 2) complicate spinning and can introduce heterogeneous microstructures that translate into radial property gradients after stabilization. Consequently, recent work emphasizes Mw in the 2×10⁵–10⁶ g·mol⁻¹ range with PDI ≈ 1.2–1.5 as a practical compromise between processability and final performance.

2.2 Comonomers and their functional role

Industrial PAN precursors are seldom homopolymers. Comonomers typically account for 0.5–6 mol% of the total composition and fulfill several roles:

• Solubility and rheology: methacrylic acid (MAA), itaconic acid (IA), acrylic acid (AA), or vinyl acetate (VA) reduce intermolecular nitrile–nitrile interactions, improving solution behavior.
• Stabilization kinetics: acidic comonomers provide sites that catalyze nitrile cyclization and mitigate uncontrolled exotherms during stabilization.
• Glass transition and mobility: comonomers disrupt regular chain packing and lower Tg, which can be helpful for drawing but may influence thermal response during stabilization.

Evidence points to IA levels around 1–2 mol% as a particularly effective choice, balancing enhanced cyclization kinetics with retention of high Mw. At these low levels, the activation energy for cyclization decreases and the onset of exothermic reactions shifts to lower temperature, but the copolymer remains robust under stabilization conditions.

By contrast, low-cost textile-grade PAN often incorporates 10 mol% or more of comonomer (e.g., sodium methallyl sulfonate, methyl acrylate). Such high functional content improves dyeability and textile performance but can complicate CF processing: the glass transition drops, and softening at relatively low temperature can lead to filament coalescence or “fiber fusion” if the stabilization schedule is not carefully controlled. Recent work shows, however, that with adapted heating profiles and tension control, even these high-comonomer grades can yield automotive-grade CF properties.

2.3 Tacticity and microstructural implications

The stereoregularity (tacticity) of PAN chains—defined by the relative arrangement of nitrile groups along the backbone—has long been discussed as a factor influencing stabilization. ¹³C NMR triad analysis is commonly used to quantify the fractions of isotactic (mm), atactic (mr) and syndiotactic (rr) sequences.

Different polymerization routes yield different tacticities:

• Aqueous redox or solution–precipitation methods often produce isotactic-enriched PAN (mm ≈ 0.3–0.4, Pm ≈ 0.5–0.6).
• Template-assisted polymerizations employing MgCl₂ or NiCl₂ can raise isotactic content further (mm > 0.5).
• Conventional commercial PAN fibers typically exhibit more moderate isotacticity.

Experimental evidence indicates that isotactic sequences favor intramolecular cyclization, leading to faster stabilization kinetics and potentially more uniform heat release. Syndiotactic-rich chains tend to encourage intermolecular crosslinking. Nevertheless, the consensus emerging from detailed spectroscopic and structural studies is that tacticity primarily affects the rate of stabilization, rather than the fundamental ladder polymer architecture or the final graphitic microstructure.

Thus, tacticity should be viewed as a lever to tune reaction kinetics and processing windows, rather than as a primary design parameter for ultimate CF performance.

3. Structural Transformations During Conventional Stabilization

3.1 Chemical pathway: from thermoplastic PAN to ladder polymer

Stabilization converts the linear PAN copolymer into a highly conjugated, infusible ladder polymer able to withstand subsequent carbonization. This process proceeds through three overlapping transformations:

1. Nitrile cyclization: neighboring –C≡N groups undergo cyclotrimerization, yielding fused six-membered rings and releasing heat (strongly exothermic).
2. Dehydrogenation: elimination of hydrogen (and associated small species) increases unsaturation and conjugation.
3. Oxidation: uptake of oxygen and formation of carbonyl and other heteroatom groups, which further crosslink the structure.

These reactions occur over a broad temperature interval, typically between 200 °C and 300 °C in air. The local kinetics depend on comonomer content, chain tacticity, molecular weight and applied tension.

3.2 Monitoring stabilization progress

Multiple techniques are used to quantify stabilization degree:

• FTIR spectroscopy: decrease of the nitrile band (~2240 cm⁻¹), growth of conjugated C=N/C=C bands (~1580–1620 cm⁻¹), and development of carbonyl peaks (~1710–1730 cm⁻¹) are routinely tracked. Various authors define stabilization indices based on these ratios.
• DSC: exothermic peaks corresponding to cyclization can be used to quantify residual reaction enthalpy and estimate remaining unreacted nitrile content.
• Density measurements: density increases from ≈1.18 g·cm⁻³ (unstabilized PAN) to ≈1.34–1.40 g·cm⁻³ as a practical indicator of adequate ladder formation.
• Elemental analysis: oxygen content typically rises into the range 8–21 wt% during effective stabilization, after which further oxidation tends to be detrimental.

A recurring finding is that fiber sets displaying the highest tensile strength after carbonization do not necessarily exhibit complete consumption of nitrile groups. Instead, “incompletely stabilized” precursors—reaching roughly 41–60% of the full cyclization extent—often yield superior strength because they avoid excessive crosslinking and oxidation that introduce internal stresses and microcracks. Fully stabilized fibers, by contrast, may favor higher modulus but are more susceptible to embrittlement.

3.3 Thermomechanical response and tension effects

Stabilization also involves substantial dimensional change under heat. Dynamic mechanical analysis (DMA) and in situ length measurements reveal:

• Viscoelastic relaxations at intermediate temperatures (<200 °C) associated with glass transition phenomena and reorganization of amorphous regions.
• A major transition near the onset of extensive cyclization (~250–270 °C), where both chemical shrinkage and relaxation occur.

If fibers are heated without tension, they tend to shrink significantly due to entropic recovery and network formation. This shrinkage reduces orientation and contributes to radial heterogeneity. Applying moderate tensile stress during stabilization counteracts these effects, maintaining fiber length and enhancing molecular alignment. However, excessive tension can cause filament breakage, especially for lower-grade or highly comonomer-rich PAN.

Industrial practice therefore relies on carefully tailored stress profiles: low to moderate tension in early stages to suppress shrinkage, sometimes relaxed slightly in later stages to accommodate chemically driven contraction without inducing damage.

4. Alternative Stabilization Strategies

The time and energy intensity of conventional stabilization have motivated exploration of alternative heating and activation methods. The core objective is to accelerate cyclization and oxidation while preserving or improving CF performance and avoiding thermal runaway.

4.1 Microwave heating and microwave plasma

Microwave heating couples electromagnetic energy directly into the material or its surroundings, yielding volumetric heating and, in plasma configurations, highly reactive species near the fiber surface.

Microwave–plasma processes have been demonstrated on PAN tows up to 24k filaments, reducing total stabilization times to about 30 minutes. The resulting carbon fibers exhibit tensile strengths on the order of 2.5–2.6 GPa and moduli compatible with automotive targets. The shortened thermal exposure also tends to reduce radial gradients and internal defects, though process control is critical to avoid localized overheating.

In some studies, microwave furnaces have been used not only for stabilization but also for carbonization. Remarkably, carbonization at around 500 °C under microwave plasma conditions has produced reported tensile strengths approaching 8 GPa and moduli near 305 GPa, with further increases in modulus up to ~655 GPa at higher temperatures. While these exceptional values depend on specific experimental conditions and may not yet be reproducible at scale, they illustrate the potential of alternative energy inputs for microstructure control.

4.2 UV-assisted stabilization

Ultraviolet irradiation can initiate radical formation in PAN, especially when comonomers or additives enhance photon absorption. UV-assisted stabilization has been applied to textile-grade PAN containing relatively high comonomer levels, with total stabilization times as low as 30 minutes.

In one representative case, UV-assisted processing of such PAN followed by conventional carbonization yielded CF with tensile strength around 2.4 GPa and modulus near 195 GPa—well within the range required for cost-sensitive structural applications. The absence of additional photoinitiators and the ability to work with inexpensive precursor grades make UV-assisted approaches attractive, though their penetration depth and uniformity must be addressed for large-tow processing.

4.3 γ-ray irradiation

γ-irradiation introduces radicals throughout the PAN fiber volume. When combined with subsequent thermal stabilization, it can enhance cyclization and reduce overall processing time. Doses in the range of 100–400 kGy have been reported.

While γ-irradiation has enabled CF strengths above 4 GPa and moduli around 245 GPa in selected studies, the technology faces practical obstacles: radiation safety, shielding requirements, and relatively low throughput in typical γ-facilities. As a result, it is more likely to remain a research tool than a mainstream industrial method.

4.4 Electron-beam irradiation (EBI)

Electron-beam irradiation is arguably the most industrially promising irradiation method for PAN stabilization. High-energy electrons generate radicals along the polymer backbone at or near room temperature. These radicals later promote rapid nitrile cyclization during moderate heat treatment.

Key observations from EBI-assisted work include:

• Stabilization degrees above 95% can be achieved at temperatures as low as 250 °C within 40 minutes.
• Carbon fibers produced from EBI-pretreated PAN display tensile strengths around 2.3 GPa for relatively short processing times.
• Hybrid routes combining EBI with plasma or controlled convection heating can further reduce total stabilization time.

A particularly noteworthy hybrid process combines approximately 10 minutes of EBI with about 3–4 minutes of plasma-assisted thermal treatment, achieving total stabilization in roughly 13 minutes. The resulting CF exhibited tensile strength around 2.4 GPa and modulus near 196 GPa. Such performance, achieved in a fraction of conventional stabilization time, underscores the potential of EBI-based strategies for high-throughput CF production.

4.5 Comparative assessment

When the various stabilization options are compared in terms of processing time, energy demand, achievable properties, and scalability, the following broad picture emerges:

• Conventional multi-step air stabilization remains the most mature and robust technology, suitable across a wide range of PAN chemistries, but offers limited scope for further time reduction without complex control schemes.
• Microwave and plasma-assisted heating provide significant time savings and can enhance microstructure, but require specialized reactors and careful process design to ensure uniform treatment of large tows.
• UV and γ-irradiation clearly demonstrate the kinetic benefits of photochemical activation, yet their industrial scalability is less straightforward.
• EBI, especially in hybrid configurations with thermal or plasma treatment, currently offers the most compelling balance of speed, controllability and potential for integration into existing processing lines, provided that suitable high-throughput irradiation facilities are available.

5. Carbonization and Development of Mechanical Properties

After stabilization, carbonization transforms the ladder polymer into a carbon-rich, turbostratic structure. The evolution of mechanical properties is controlled by both the quality of the stabilized precursor and the details of the carbonization schedule.

5.1 Temperature and dwell time

Carbonization is conventionally performed in nitrogen or inert atmosphere, typically with:

• a first stage around 400–800 °C to remove non-carbon elements (H, O, excess N), and
• a high-temperature stage up to 1200–1500 °C for standard modulus fibers, or up to 1800–2500 °C when high modulus is desired.

Across many studies, tensile strength tends to reach a maximum in the range 1200–1500 °C. Beyond this, crystallite growth and defect coarsening often lead to strength reduction, even as modulus continues to increase with improved graphitic order.

5.2 Influence of precursor quality and stabilization level

The radial and axial uniformity achieved during stabilization has a direct impact on strength. Incompletely stabilized fibers with under-converted cores can develop voids or weak interfaces during carbonization, while over-oxidized fibers may contain microcracks and embrittled regions.

Data compilations show that CFs derived from precursors stabilized to a moderate degree (≈41–60% cyclization, oxygen ≈8–21 wt%) often deliver higher strength than those from heavily stabilized fibers, provided that no fusible regions remain. For modulus, more extensive stabilization and higher carbonization temperatures are beneficial, as they favor ordered carbon formation and reduce residual disorder.

5.3 Alternative carbonization methods

Microwave and microwave plasma carbonization have already been mentioned as promising for both time reduction and microstructure control. The combination of volumetric heating and reactive plasma species can accelerate defect healing and surface structuring. Some of the highest reported strengths and moduli for PAN-based CFs originate from such routes, although reproducibility and scalability remain under active investigation.

Other enhancements—such as applying external magnetic fields during heat treatment or introducing dopants (e.g., boron) to catalyze graphitization—have shown more modest improvements in modulus relative to the additional complexity they introduce. For large-scale industrial deployment, priorities currently center on more energy-efficient furnaces, improved tension control, and minimized gas treatment costs rather than exotic field-assisted methods.

6. Cost, Sustainability, and the Role of Low-Cost Precursors

6.1 Cost structure and alternative PAN grades

Techno-economic analyses consistently indicate that PAN precursor and stabilization dominate CF cost. Moving from specialty CF-grade PAN to lower-cost grades, including textile PAN or melt-spun PAN with higher comonomer content, can substantially reduce precursor cost per kilogram.

The core question is how much performance must be sacrificed, and whether advanced stabilization and carbonization can compensate. The surveyed literature suggests:

• PAN with higher comonomer content can still reach tensile strengths around 2–2.5 GPa and moduli near 180–200 GPa when stabilized and carbonized under optimized conditions.
• For many automotive, wind, and infrastructure applications, such properties are adequate; the limiting factors become design and certification, not material performance alone.

Alternative stabilization methods (UV, EBI, microwave plasma) are particularly attractive for these lower-grade precursors, because they can shorten exposure in temperature regimes where fusion and sticking are most likely, while artificially driving cyclization through radical or plasma activation.

6.2 Environmental and energy considerations

From a sustainability perspective, several aspects are noteworthy:

• Energy demand: Stabilization accounts for a large portion of cumulative energy demand. Alternative strategies that reduce stabilization times from hours to tens of minutes can cut this substantially.
• Emissions: Conventional stabilization and carbonization of PAN produce off-gases (e.g., CO₂, CO, HCN, NH₃) that require treatment or incineration. Irradiation-assisted and lower-temperature processes can potentially reduce peak temperatures and associated emissions, but detailed life-cycle studies are still limited.
• Precursor sourcing: The use of melt-spun or textile PAN may leverage existing large-scale polymer production and spinning infrastructure. When coupled with bio-based acrylonitrile or recycled PAN streams, further improvements in life-cycle performance are possible.

While the reviewed work supports the notion that PAN-based CF can be made more eco-efficient through process intensification, it also underlines the practical barriers: capital cost for irradiation facilities, safety regulations, and the need for robust acid- or gas-handling systems where applicable. Any holistic assessment must weigh these factors against energy and cost savings.

7. Outlook and Future Research Directions

The body of evidence assembled around PAN precursor design and alternative stabilization technologies points to several clear directions for further development:

1. Integrated precursor–process design
   Future work should treat polymer synthesis, spinning, stabilization, and carbonization as a coupled system. High Mw, narrow PDI PAN with carefully chosen acidic comonomer levels needs to be matched to stabilization and carbonization profiles that exploit its kinetic and mechanical advantages. Conversely, textile-grade PAN must be paired with process routes that minimize low-temperature softening and fusion.
2. Quantitative stabilization metrics
   While density, oxygen content and FTIR indices are widely used, there is a need for standardized, quantitative descriptors of stabilization degree that correlate reliably with CF properties across different PAN chemistries and process routes. Advanced in situ techniques—such as two-dimensional FTIR mapping, synchrotron X-ray scattering, or in-line spectroscopic monitoring—could support this.
3. Scaling irradiation-assisted stabilization
   Electron-beam and microwave/plasma techniques have shown compelling results at laboratory and pilot scales. The next step is to demonstrate stable, uniform treatment of industrial tow sizes (24k–50k and beyond) at line speeds compatible with large-volume production. This will require progress in reactor design, beam control, thermal management, and process monitoring.
4. Microstructure–property relationships in fast-stabilized fibers
   Many irradiation-accelerated processes generate distinct microstructures compared with conventional stabilization. Systematic characterization—linking local ladder polymer topology, defect distribution, and carbonization behavior to final CF strength and modulus—is essential to establish robust design rules, especially when targeting ultra-high modulus or very high strength.
5. Balanced sustainability assessments
   Eco-efficiency claims for alternative stabilization technologies should be supported by full life-cycle assessments that include capital and operating impacts of irradiation equipment, gas treatment, and any additional consumables. Similarly, the environmental benefits of using melt-spun or textile-grade PAN must be weighed against potential increases in scrap rate or lower CF performance.

In summary, the combination of advanced PAN chemistry, judicious use of comonomers, and innovative stabilization technologies offers a credible route to high-performance CFs with reduced cost and environmental burden. Incomplete but well-controlled stabilization, coupled with appropriate carbonization, appears to be a key concept for achieving high tensile strength without excessive processing time. As irradiation-assisted methods move from laboratory experiments toward industrial implementation, they may substantially reshape the cost and performance landscape of PAN-based carbon fibers.

Representative References

1. Soulis, S.; Konstantopoulos, G.; Koumoulos, E. P.; Charitidis, C. A. Impact of Alternative Stabilization Strategies for the Production of PAN-Based Carbon Fibers with High Performance. Fibers 2020, 8(6), 33.
2. Nunna, S.; et al. Investigation of the Progress of Reactions and Evolution of Radial Heterogeneity in the Initial Stage of Thermal Stabilization of PAN Precursor Fibres. Polymer Degradation and Stability 2016, 125, 105–114.
3. Morris, E. A.; et al. High Performance Carbon Fibers from Very High Molecular Weight Polyacrylonitrile Precursors. Carbon 2016, 101, 245–252.
4. Kim, S. Y.; et al. Microwave Plasma Carbonization for the Fabrication of Polyacrylonitrile-Based Carbon Fiber. Polymer 2015, 56, 590–595.
5. Tanaka, F.; Okabe, T. Historical Review of Processing, Microstructures, and Mechanical Properties of PAN-Based Carbon Fibers. Reference Module in Materials Science and Materials Engineering 2018.