Particle size distribution (PSD) determination of lignin-rubber pyrolysis products is essential for evaluating the reinforcement performance, processability, and commercial value of carbonaceous residues recovered from lignin-filled rubber materials. This article explains the scope of testing, typical sample types, measured parameters, internationally recognized methods such as laser diffraction and dynamic light scattering, and practical applications across the rubber, recycling, and sustainable materials industries.
Technical Overview
Lignin is one of the most abundant natural aromatic polymers and is generated in large volumes as a byproduct of the pulping and biorefinery industries. In recent years, lignin has attracted significant attention as a renewable bio-filler that can partially or fully replace conventional carbon black in rubber formulations, reducing the reliance on fossil-based reinforcing agents while improving the sustainability profile of elastomer products.
Pyrolysis is a thermochemical conversion process in which lignin-rubber composites, vulcanizates, or end-of-life rubber goods are heated in an oxygen-free or oxygen-limited environment. The process yields three product streams: pyrolysis oil, non-condensable gas, and a solid carbonaceous residue. The solid fraction—often described as lignin-rubber pyrolysis char or recovered carbonaceous filler—retains a structure that depends strongly on the lignin content, pyrolysis temperature, heating rate, and residence time employed during decomposition.
Particle size distribution (PSD) is a critical quality attribute of these pyrolysis products because it directly governs rubber-filler interaction, dispersion behavior, compound viscosity, mechanical reinforcement, and downstream processability. Accurate PSD determination therefore supports product development, process optimization, batch-to-batch quality control, and the qualification of recovered fillers for reuse in new rubber goods. Accredited third-party laboratories routinely perform these measurements in accordance with internationally recognized standards such as ISO 13320, ISO 22412, ISO 13322, and their national equivalents.
Testing Samples
Lignin-Rubber Pyrolysis Char (Solid Residue)
The primary sample type is the solid residue collected after pyrolysis of lignin-filled rubber compounds or vulcanized rubber articles containing lignin-based fillers. The char is typically cooled under inert atmosphere, milled or gently ground, and sieved to a defined fraction before analysis. Sample heterogeneity, residual inorganic ash, and residual steel or mineral contaminants must be considered during sub-sampling.
Recovered Carbon Black Substitute from Lignin-Rubber Waste
End-of-life rubber products in which lignin has partially replaced carbon black can be processed through industrial pyrolysis to recover a carbonaceous filler intended for reuse. These recovered fillers require PSD verification to confirm equivalence with conventional furnace carbon black grades and to demonstrate suitability for semi-reinforcing or reinforcing applications.
Lignin-Derived Carbon and Activated Carbon Precursors
Lignin-rich pyrolysis residues intended for further upgrading—such as steam or chemical activation for activated carbon, or graphitization for electrochemical applications—are characterized at intermediate process stages to monitor how thermal treatment modifies particle dimensions and agglomeration behavior.
Raw Lignin and Compound Precursor Powders
Feedstock characterization is equally important. Technical lignins (kraft, soda, organosolv, hydrolysis lignin) and pre-compounded lignin-rubber masterbatch powders are analyzed to establish baseline particle size data, enabling correlation between precursor PSD and final pyrolysis product morphology.
Testing Items
Typical reporting parameters for particle size distribution determination of lignin-rubber pyrolysis products include:
- Characteristic diameters: D10, D50 (median), and D90 on a volume-weighted or number-weighted basis
- Mean particle size (volume mean, surface-weighted mean, and number mean diameters)
- Distribution width indicators: span value (D90 minus D10 divided by D50), uniformity index, and modal size
- Volume or mass percentage within specified size classes or cumulative undersize/oversize curves
- Fines fraction below a defined threshold (for example, particles smaller than 1 micrometer or 10 micrometers)
- Coarse fraction and oversize agglomerate content above defined cutoff points
- Shape and morphology descriptors when image-based methods are applied (aspect ratio, circularity, elongation)
- Estimated specific surface behavior derived from PSD data, often cross-referenced with BET surface area results
Testing Methods
Laser Diffraction Analysis
Laser diffraction is the most widely used technique for PSD determination of pyrolysis chars and recovered fillers, covering a broad measurement range typically from approximately 0.1 micrometers to 3 millimeters. The method is performed in accordance with ISO 13320 or equivalent national standards such as GB/T 19077. Both wet dispersion (in water or isopropanol with a suitable nonionic surfactant) and dry dispersion (compressed air jet dispersion) modes may be used; selection depends on the fragility of agglomerates and the moisture sensitivity of the sample. Mie optical theory is applied using realistic refractive index values for carbonaceous materials, and method validation includes verification of dispersion adequacy and obscuration stability.
Dynamic Light Scattering
For submicron and nanoscale fractions—such as colloidal carbon formed during flash pyrolysis or fine primary particles detached from agglomerates—dynamic light scattering is applied following ISO 22412 or GB/T 19627. This method reports the intensity-weighted hydrodynamic diameter and polydispersity index, providing complementary insight into the fine end of the distribution where laser diffraction resolution is limited.
Image Analysis and Electron Microscopy
Static image analysis per ISO 13322-1 and dynamic image analysis per ISO 13322-2 provide number-based size distributions together with particle shape information, which is valuable for distinguishing spherical carbon blacks from irregular lignin-derived char structures. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM), following the principles of ASTM D3849 for carbon black morphological characterization, are used to determine primary aggregate and agglomerate dimensions and to confirm the dispersion state observed in ensemble techniques.
Sieving and Supplementary Techniques
Mechanical sieve analysis remains applicable for coarse fractions above approximately 45 to 75 micrometers, particularly for quality screening of milled char. Electrical sensing zone (Coulter principle) measurements and sedimentation-based methods may be used as supplementary or referee techniques where required by specification.
Sample Preparation and Dispersion Control
Representative sampling and controlled dispersion are decisive for valid results. Samples are quartered or riffle-split to obtain statistically representative test portions, dried to constant mass at low temperature to avoid oxidation, and dispersed following guidance such as ISO 14887. Controlled ultrasonication is applied with documented energy input to break weak agglomerates without fracturing primary particles, and dispersion conditions are verified by repeated measurement until a stable distribution is achieved.
Data Representation and Reporting
Results are expressed according to ISO 9276-1, including tabulated percentiles, histogram and cumulative curves, measurement conditions, dispersion medium, refractive index assumptions, and instrument repeatability. Reports issued by accredited third-party laboratories typically include measurement uncertainty and traceability information to support regulatory submission and customer qualification.
Applications
- Development and qualification of lignin-based reinforcing fillers as sustainable substitutes for conventional carbon black in tire and industrial rubber goods
- Quality control of recovered carbonaceous fillers produced by pyrolysis of lignin-rubber waste streams destined for reuse in new rubber compounds
- Optimization of pyrolysis process parameters—temperature, heating rate, residence time, and feed lignin loading—by correlating operating conditions with resulting particle size and agglomeration behavior
- Prediction of rubber compound processability, Mooney viscosity, dispersion rating, and tensile or abrasion performance from filler PSD data
- Upgrading of lignin pyrolysis chars into activated carbons, battery anode precursors, and pigment-grade carbon materials requiring tight size specifications
- Support for environmental certification, circular economy documentation, and technical data sheets for commercial bio-derived filler products
- Comparative benchmarking of lignin-rubber pyrolysis products against standard carbon black grades such as N300, N500, N600, and N700 series materials
FAQ
Why is particle size distribution so important for lignin-rubber pyrolysis products?
The PSD of the recovered carbonaceous filler determines its contact area with the rubber matrix, its dispersion characteristics during mixing, and consequently the mechanical reinforcement and dynamic properties of the final compound. Distributions that are too coarse or too broad lead to poor dispersion, weak filler-rubber interaction, and degraded product performance, so PSD acts as a primary acceptance criterion in both development and trade.
Which measurement technique should be selected for a given sample?
Laser diffraction per ISO 13320 is the default choice for the wide size range typical of pyrolysis chars. Dynamic light scattering per ISO 22412 is added when submicron fractions must be quantified, while image analysis per ISO 13322 and electron microscopy per ASTM D3849 principles are used when shape information or verification of primary particle morphology is required. Many accredited third-party laboratories apply a combination of these methods for full characterization.
How are agglomerates handled during testing?
Agglomerates are common in pyrolysis chars due to residual volatiles and van der Waals forces. Laboratories apply controlled wet or dry dispersion with defined ultrasonic energy, verify that repeated measurements are stable, and report both the as-received agglomerated distribution and the dispersed distribution so that clients can distinguish primary particle size from soft agglomeration.
How much sample is required and what is the typical turnaround time?
For laser diffraction, a representative test portion of roughly 0.5 to 5 grams is usually sufficient, while microscopy and DLS require only milligram quantities. Standard turnaround is commonly three to seven working days, extending to ten to fifteen working days when method development, dispersion optimization, or multi-technique confirmation is involved.
Which standards govern the reporting of results?
Reporting follows ISO 9276-1 for graphical and tabular representation of particle size analysis results, with underlying measurement standards including ISO 13320, ISO 22412, ISO 13322-1, ISO 13322-2, ISO 14887, ASTM D3849, and corresponding national standards such as the GB/T 19077 and GB/T 19627 series.