Sizing agent content is a critical quality parameter that governs the handleability, wettability, and fiber-matrix adhesion of pitch-based carbon fibers in advanced composites. Accurate determination of size content—typically ranging from 0.3% to 2.0% by mass—is performed using standardized gravimetric, extraction, and thermal methods, most notably those described in ISO 10548. This article presents a comprehensive technical overview of sample types, test items, analytical methods, and industrial applications related to sizing agent content testing of pitch-based carbon fibers.
Technical Overview
Pitch-based carbon fiber is manufactured from petroleum pitch or coal tar pitch precursors and is available in two principal grades: mesophase pitch fiber, which offers ultra-high modulus and exceptional thermal conductivity, and isotropic pitch fiber, which provides a cost-effective solution for general-purpose applications. Because the as-produced graphitic surface of pitch-based fiber is smooth, chemically inert, and poorly reactive toward polymer matrices, a thin organic coating known as a sizing agent is applied immediately after carbonization and surface treatment.
The sizing agent serves multiple essential functions. It protects individual filaments from abrasion during spooling, weaving, winding, and chopping; it bundles filaments into a coherent tow that resists fuzzing and filament breakage; it lubricates the fiber during textile processing; and it acts as a coupling layer that promotes wetting and chemical bonding with thermoset or thermoplastic matrix resins. Epoxy-based sizes are the most common, although thermoplastic-compatible, phenoxy, polyurethane, and hybrid sizes are also used depending on the target composite system.
Because sizing content directly influences composite interlaminar shear strength, filament damage rates, impregnation quality, and mechanical performance, precise quantification is required for incoming material inspection, production release testing, and sizing formulation development. Insufficient sizing leads to fiber damage and poor wet-out, while excessive sizing dilutes the matrix, creates a weak interphase, and degrades high-temperature performance—particularly critical for pitch fiber grades intended for thermal management and aerospace structures.
Testing Samples
Continuous Tow and Yarn
Continuous pitch-based carbon fiber tow, commonly supplied in 1K, 2K, 3K, 6K, or 12K filament counts, is the primary sample form for sizing content determination. Specimens of approximately 2 to 5 grams are cut from spools at specified positions—beginning, middle, and end of the wind—to evaluate sizing uniformity along the production length. All specimens must be conditioned in a controlled environment, typically 23 ± 2 °C and 50 ± 5% relative humidity, before weighing.
Chopped Fiber
Chopped pitch-based carbon fiber with staple lengths of 3 to 12 mm is sampled from homogenized bulk lots. Because chopped fiber can segregate during packaging, multiple subsamples are drawn from different locations and combined to form a representative test specimen, ensuring the reported result reflects the entire lot.
Woven Fabric and Preforms
Pitch-based carbon fiber fabrics, tapes, and braided preforms are sampled by cutting coupons from designated zones across the fabric width. Warp and weft yarns may carry different sizing loads, so sampling plans must account for directionality when characterizing woven materials.
Milled and Short Fibers
Milled and short-cut pitch fibers are frequently supplied in an unsized state, particularly for friction materials and thermal insulation. Testing of these products focuses on verifying the absence or residual trace level of sizing agent, confirming effective desizing, or quantifying process coatings applied after milling.
Testing Items
A comprehensive sizing characterization program for pitch-based carbon fiber typically includes the following test items:
- Total sizing content, expressed as mass percentage on a dry fiber basis
- Sizing uniformity along the tow length and between spools or production lots
- Moisture content and volatile content, which must be separated from true sizing mass loss
- Sizing agent type identification and chemical confirmation by infrared spectroscopy
- Desizing efficiency verification for fibers intended to be used unsized
- Surface energy and wettability assessment through contact angle measurement
- Correlation testing with composite interlaminar shear strength for interphase quality evaluation
Testing Methods
Solvent Extraction Method
The solvent extraction method is the most widely applied technique and forms the basis of ISO 10518-family procedures, most notably ISO 10548 for the determination of size content of carbon fiber. A pre-weighed dry specimen is refluxed in a hot solvent—acetone, tetrahydrofuran, dimethylformamide, or a tailored solvent system selected according to the size chemistry—for 4 to 12 hours until the extracted mass reaches constant weight. After extraction, the specimen is rinsed with fresh solvent, dried at 105 °C to constant mass, and reweighed. The sizing content is calculated as: sizing content (%) = (m0 − m1) / m0 × 100, where m0 is the original dry mass and m1 is the extracted dry mass. This method is non-destructive to the fiber and permits subsequent mechanical testing of the desized specimen.
Acid Digestion Method
For partially cured, crosslinked, or otherwise solvent-resistant sizes, the acid digestion method provides more complete removal. The specimen is treated with hot concentrated sulfuric acid, nitric acid, or a mixed acid system that oxidatively decomposes the organic size while leaving the highly graphitic pitch fiber structure essentially intact under controlled conditions. After digestion, the residue is thoroughly washed to neutrality with deionized water, dried, and weighed. This method is particularly suitable for pitch fibers carrying thermoset sizes and is specified as an alternative procedure within ISO 10548.
Furnace Pyrolysis Method
The furnace burn-off method determines sizing content by pyrolyzing the organic coating in a muffle furnace, typically at 400 to 450 °C in air for a short, strictly controlled residence time. Because pitch-based carbon fiber begins to oxidize above approximately 450 to 500 °C, precise temperature control and calibration against a desized reference are mandatory to avoid overestimating sizing content through fiber mass loss. This gravimetric approach is rapid and suited to high-volume quality control screening.
Thermogravimetric Analysis Method
Thermogravimetric analysis offers a micro-scale complementary technique requiring only a few milligrams of fiber. The specimen is heated under nitrogen, followed by a switch to an oxidizing atmosphere; the discrete mass-loss step occurring between approximately 300 and 600 °C is attributed to the sizing agent. TGA is especially valuable in research and development for comparing size formulations, assessing thermal stability, and cross-validating bulk gravimetric results.
Supporting Surface Characterization
In addition to gravimetric quantification, accredited third-party laboratories commonly apply supporting techniques to fully characterize the sized fiber surface:
- Scanning electron microscopy to evaluate size distribution, filament damage, and surface morphology
- Fourier-transform infrared spectroscopy to identify size chemistry and confirm complete removal after desizing
- X-ray photoelectron spectroscopy to quantify surface elemental composition and functional group density
- Dynamic contact angle measurement to determine surface energy and predict resin wetting behavior
Applications
Sizing agent content testing of pitch-based carbon fiber supports quality assurance and product development across a broad range of high-performance industries:
- Aerospace thermal management structures, including satellite radiator panels, heat sinks, and thermal spreaders, where controlled sizing ensures reliable bonding of high-conductivity pitch fibers into aluminum, copper, or resin matrices
- Carbon-carbon composite production for aircraft brake discs and rocket nozzle components, where sizing control affects densification behavior and interfacial integrity
- High-modulus sporting goods such as golf shafts, fishing rods, and bicycle components that exploit the stiffness of mesophase pitch fiber
- Robotics, optical benches, and precision structures requiring dimensional stability combined with vibration damping
- Friction materials, gaskets, and cement reinforcement using isotropic pitch fiber, where residual size verification governs downstream compatibility
- Internal quality control laboratories of fiber manufacturers and incoming inspection programs at composite fabricators
FAQ
What is the typical sizing content of pitch-based carbon fiber?
Commercial pitch-based carbon fibers generally carry between 0.3% and 2.0% sizing agent by mass, with epoxy-compatible grades most commonly falling in the 0.7% to 1.2% range. Unsized grades and milled fibers are verified to contain residual size below approximately 0.05%.
Which test method should be selected?
Method selection depends on size chemistry and test purpose. Solvent extraction is preferred for removable sizes and allows subsequent fiber testing; acid digestion is more effective for crosslinked thermoset sizes; furnace burn-off suits rapid screening; and TGA is ideal for research-level cross-validation. ISO 10548 recognizes extraction and digestion as the reference procedures.
How much sample is required and how is precision ensured?
A typical program requires 5 to 10 grams of fiber per specimen with a minimum of three replicate determinations. Moisture is determined separately or eliminated by pre-drying, and results are reported on a dry-mass basis with calculated standard deviation.
What accuracy can be expected?
With proper conditioning and calibrated balances, gravimetric methods achieve repeatability within approximately ±0.05% absolute sizing content. Agreement between orthogonal methods, such as extraction versus TGA, provides the highest confidence in reported values.
What is the typical turnaround time?
Standard sizing content determinations at accredited third-party laboratories are generally completed within 3 to 7 working days, depending on the selected method, replicate count, and whether supporting spectroscopic or surface characterization is requested.
