Particle size analysis of carbon fiber pyrolysis products in cosmetics is essential for confirming formulation performance, evaluating safety, and demonstrating compliance with international nanomaterial regulations. This technical article reviews the relevant sample types, measurable parameters, and validated analytical methods—including laser diffraction, dynamic light scattering, centrifugal sedimentation, and electron microscopy—for characterizing pyrolyzed carbonaceous powders used in cosmetic formulations.
Technical Overview
Carbon fiber pyrolysis products are carbonaceous materials obtained by the controlled thermal decomposition of carbon fiber or carbon fiber-reinforced polymer feedstocks under inert or oxygen-limited atmospheres, typically at temperatures between 500 °C and 1000 °C. In cosmetics, these recycled or engineered carbon powders may serve as black colorants in mascaras and eyeliners, adsorbent fillers in cleansing masks, sebum-control agents in primers, and sensory modifiers in various leave-on and rinse-off products. Because the functional behavior and toxicological profile of these materials are governed by particle size, morphology, and dispersion state, rigorous particle size analysis is a cornerstone of quality control and safety assessment.
Several factors make this analysis technically demanding. Pyrolyzed carbon materials are often highly polydisperse, combining granular char particles, fibrous fragments, and platelet-like graphitic structures. Elemental carbon strongly absorbs visible light, which complicates optical measurement models, and its hydrophobic surface promotes agglomeration in aqueous media. In addition, finished cosmetic formulations contain oils, waxes, surfactants, and polymers that must be removed without altering the native particle size distribution. For these reasons, orthogonal measurement techniques and carefully validated sample preparation procedures are required.
Particle size data also carry direct regulatory consequences. Under the EU Cosmetics Regulation (EC) No 1223/2009, an insoluble or biopersistent material is classified as a nanomaterial when 50% or more of its particles, in a number-based size distribution, have one or more external dimensions between 1 nm and 100 nm, triggering pre-market notification obligations. The Scientific Committee on Consumer Safety (SCCS) guidance on nanomaterials further emphasizes number-based characterization. Fibrous fragments must additionally be assessed against the WHO fiber convention (length greater than 5 µm, diameter less than 3 µm, aspect ratio of 3 or more), because biopersistent fibers raise specific inhalation-toxicology concerns during both manufacturing and use.
Testing Samples
Raw Materials and Intermediates
Incoming pyrolyzed carbon fiber powders, milled recycled carbon fiber, pyrolytic char recovered from composite recycling processes, and concentrated masterbatch dispersions supplied to cosmetic manufacturers are analyzed to establish material specifications, verify supplier consistency, and screen for unexpected nanoscale or fibrous fractions.
Finished Cosmetic Products
Finished products such as mascaras, eyeliners, color cosmetics, cleansing and purifying masks, sebum-control primers, soaps, and exfoliating products are tested to confirm that the particle size distribution in the final formulation remains within specification after processing, shearing, and storage.
Extracted and Fractionated Samples
Particles recovered from cosmetic matrices through validated extraction procedures are analyzed to obtain the native size distribution free of matrix interference. Size-fractionated samples, such as sub-100 nm cuts or respirable dust fractions, may also be prepared to support toxicological evaluation and occupational exposure assessment.
Testing Items
Depending on the purpose of the study, particle size analysis of carbon fiber pyrolysis products typically covers the following parameters:
- Volume-based and number-based particle size distributions, including D10, D50, D90, and D97 values and distribution span
- Number-based nanofraction, expressed as the percentage of particles smaller than 100 nm in at least one external dimension
- Fiber length distribution, fiber diameter distribution, and aspect ratio statistics for fibrous fragments
- Count concentration of WHO-classifiable fibers (length greater than 5 µm, diameter less than 3 µm, aspect ratio of 3 or more)
- Agglomerate size versus primary particle size after defined dispersion energy input
- Shape descriptors such as circularity, elongation, and roughness derived from image analysis
- Specific surface area by gas adsorption, which provides an indirect average particle size check
- Zeta potential and dispersion stability in representative formulation media
- Particle mass concentration and count concentration in recovered extracts or airborne dust samples
Testing Methods
Sample Preparation and Matrix Removal
Representative sampling and gentle dispersion are prerequisites for meaningful results. Powders are dispersed in suitable liquid media following the general principles of ISO 14887, using selected nonionic or anionic dispersants and documented ultrasonication energy to break weak agglomerates without fragmenting fibers. For finished products, organic matrices are removed by solvent extraction and repeated centrifugal washing, followed by dialysis or redispersion; recovery rates are verified through spike-and-recovery experiments to confirm that the preparation itself does not shift the size distribution.
Laser Diffraction
Laser diffraction performed in accordance with ISO 13320 and its national equivalent GB/T 19077 covers an approximate range of 0.1 µm to 3500 µm and delivers robust volume-based D-values for routine quality control. Because elemental carbon strongly absorbs light, an appropriate Mie optical model with realistic absorption behavior, or the Fraunhofer approximation, must be applied. For fibrous materials, results should be reported as equivalent spherical diameters and interpreted with caution.
Dynamic Light Scattering and Nanoparticle Tracking Analysis
Dynamic light scattering following ISO 22412 and GB/T 19627 measures the hydrodynamic diameter of the colloidal fraction from roughly 1 nm to 1 µm and is an efficient screening tool for nanoscale content. Nanoparticle tracking analysis provides number-based size distributions and approximate particle concentrations in the same range. Both techniques are intensity- or trace-weighted and should be confirmed by independent number-based methods before regulatory conclusions are drawn.
Centrifugal Sedimentation
Line-start disc centrifugation based on the principles of ISO 13318 resolves particles from a few nanometers to several micrometers and is particularly effective for dense carbonaceous powders. It offers excellent resolution of the sub-100 nm region and supports quantification of the nanofraction independently of optical light-scattering artifacts.
Electron Microscopy and Image Analysis
Scanning electron microscopy and transmission electron microscopy, combined with static image analysis per ISO 13322-1, provide direct visualization of primary particles, agglomerates, and fibrous fragments, and they remain the reference approach for number-based distributions in the nanoscale regime. Fiber length and diameter distributions are measured in accordance with ISO/TS 11937, with statistically sufficient particle counts and calibrated pixel scales. Dynamic image analysis per ISO 13322-2 supplements these methods for rapid morphological screening of larger fragments.
Ancillary and Confirmatory Techniques
Specific surface area determined by gas adsorption under ISO 9277 provides a consistency check on average particle size. For inhalation-related assessments, cascade impaction characterizes the aerodynamic size of airborne dust generated during handling, while differential mobility analysis under the ISO 15900 series addresses aerosolized nanoparticles. The evaluation strategy for fibrous nanomaterials can be structured according to ISO/TR 22293. It should be noted that single-particle inductively coupled plasma mass spectrometry is generally not applicable to pure elemental carbon due to the absence of a suitable measurable isotope, so number-based nanofraction determination relies on electron microscopy, nanoparticle tracking analysis, and centrifugal sedimentation.
Data Quality and Method Validation
All measurements are supported by method validation covering precision, bias, and measurement uncertainty, with calibration checks against certified polydisperse reference materials. Data presentation and distribution calculations follow the ISO 9276 series, and studies intended for regulatory submission are conducted by accredited third-party laboratories operating under ISO/IEC 17025.
Applications
Particle size analysis of carbon fiber pyrolysis products supports the complete lifecycle of cosmetic ingredient and product development:
- Regulatory compliance, including EU nanomaterial notification through the Cosmetic Products Notification Portal, SCCS dossier preparation, and safety assessment submissions to national authorities
- Formulation optimization, balancing color intensity, matte or glossy finish, spreadability, and adsorption performance
- Incoming material inspection and supplier qualification against defined D50, D90, and span specifications
- Batch release testing and stability monitoring across shelf-life and stress-storage conditions
- Toxicological risk assessment, including respirable dust characterization and fiber-count screening for occupational exposure control
- Claim substantiation for sebum adsorption, purifying, and long-wear performance claims
FAQ
What data are required to classify a carbon-based cosmetic ingredient as a nanomaterial in the EU?
A validated number-based particle size distribution is required to determine whether at least 50% of particles have one or more external dimensions between 1 nm and 100 nm. Electron microscopy-based counting is the reference method, supported by centrifugal sedimentation or nanoparticle tracking analysis. Intensity-weighted screening data alone are not sufficient for classification.
Which method is most suitable for fibrous pyrolysis fragments?
Scanning and transmission electron microscopy per ISO/TS 11937 provide direct measurements of fiber length, diameter, and aspect ratio, and enable counting of WHO-classifiable fibers. Laser diffraction may be used for routine monitoring, but its equivalent spherical diameter output does not capture fiber geometry.
How are particles recovered from emulsion-based cosmetic products?
Organic matrices are removed through solvent extraction followed by centrifugal washing and dialysis, using conditions mild enough to avoid oxidizing the carbon surface or breaking fibers. Recovery is verified by spike-and-recovery testing before the extracted distribution is accepted as representative.
Why do wet and dry laser diffraction results sometimes differ for the same powder?
Differences arise from the dispersion state of agglomerates, the optical model applied to strongly absorbing carbon, and the shear history of the sample. Both conditions should be documented, and dispersion energy should be reported alongside the results to make the data comparable.
What specifications are typically set for quality control?
Specifications are formulation- and supplier-specific, but common parameters include D50 and D90 limits, a distribution span ceiling, and a defined maximum nanofraction or fiber-count threshold where regulatory or safety considerations apply.