Crystallinity determination of lignin-rubber pyrolysis products is a critical analytical step for evaluating the structural order of recovered carbonaceous materials, including char and recovered carbon black derived from lignin-filled rubber composites. This article outlines the sample types, key test parameters, X-ray diffraction (XRD) and thermal analysis methodologies, and practical applications of crystallinity measurement in rubber recycling and bio-based material development. All testing approaches described follow internationally recognized standards from ISO and ASTM and can be performed by accredited third-party laboratories.
Technical Overview
Lignin-rubber composites combine natural or synthetic rubber matrices with lignin, a renewable aromatic biopolymer used as a partial replacement for carbon black filler. When these composites reach end-of-life, pyrolysis converts them into solid carbonaceous residues (char), pyrolysis oil, and fuel gas. The degree of crystallinity of the solid pyrolysis products is a decisive structural parameter: it governs reinforcing performance when the char is reused as recovered carbon black, controls adsorption capacity and electrical conductivity, and reflects the extent of carbonization and incipient graphitic ordering achieved during thermal decomposition.
Because lignin-rubber pyrolysis chars are structurally heterogeneous—containing turbostratic carbon, amorphous carbon, residual inorganic ash (zinc oxide, silica, zinc sulfide), and possible semi-crystalline organic fractions—crystallinity determination requires careful method selection and sample preparation. Wide-angle X-ray diffraction (WAXD/XRD) is the primary technique, supported by differential scanning calorimetry (DSC) for any fusible crystalline fraction and Raman spectroscopy as a complementary tool for assessing carbon structural order. Quantitative results are typically expressed as a crystallinity index (CrI), an overall degree of crystallinity in percent, crystallite dimensions, and interlayer d-spacings.
Testing Samples
Solid Pyrolysis Char and Biochar
The carbonaceous solid residue recovered after pyrolysis of lignin-filled natural rubber, styrene-butadiene rubber (SBR), or butadiene rubber compounds. Typical particle size for submission is a fine powder milled below 75–150 micrometers, dried to constant mass at 105 °C to eliminate moisture-related artifacts.
Recovered Carbon Black (rCB)
Refined and pelletized carbon black substitutes obtained from the pyrolysis of lignin-rubber vulcanizates, including waste tire formulations incorporating lignin biofiller. These samples often require demineralization (acid washing) or ash correction before meaningful crystallinity values can be reported.
Demineralized Carbon Residues
Acid-washed pyrolysis residues from which zinc oxide, silica, and other inorganic compounding residues have been removed. Demineralized samples allow the crystalline contribution of the carbon phase itself to be isolated and quantified without interference from sharp inorganic diffraction peaks.
Reference Materials and Precursor Samples
Pure lignin pyrolyzed under identical temperature-programmed conditions, unpyrolyzed ground rubber vulcanizates, and commercial carbon black reference grades may be submitted alongside the target samples to establish comparative baselines for crystallinity evolution studies.
Testing Items
Core measurable parameters for crystallinity characterization of lignin-rubber pyrolysis products include:
- Overall degree of crystallinity (mass fraction, %) of the carbon phase
- Crystallinity index (CrI) calculated by the Segal empirical method
- Crystallinity obtained by peak deconvolution of the (002) carbon reflection
- Amorphous fraction and turbostratic carbon contribution
- Interlayer spacing d(002) and d(100), indicative of carbon layer stacking order
- Crystallite height Lc (perpendicular to the basal planes) and crystallite diameter La derived from the Scherrer equation
- Apparent graphitization degree and Raman ID/IG band intensity ratio as a supplementary ordering indicator
- Melting and crystallization enthalpies of any residual semi-crystalline organic fraction by DSC
- Ash content for normalization of crystallinity values to the organic carbon basis
Testing Methods
X-Ray Diffraction (XRD) — Primary Method
Powder XRD is performed using Cu K-alpha radiation, typically scanning 2-theta from 5° to 80° with a step size of 0.01–0.05°. The amorphous halo centered near 20–24° 2-theta and the crystalline (002) graphite-like reflection near 24–26° 2-theta are separated by profile fitting. The degree of crystallinity is calculated from the ratio of the integrated crystalline peak area to the total scattering area. Two well-established calculation routes are offered:
- Segal method: a fast empirical calculation of the crystallinity index from peak intensities
- Ruland or Hermans–Weidinger methods: rigorous area-based approaches accounting for disorder broadening, preferred for publication-grade data
Differential Scanning Calorimetry (DSC)
DSC following the ISO 11357 series (including Part 3 on temperature and enthalpy of melting and crystallization) and ASTM D3418 / ASTM E1356 is applied to any fusible or semi-crystalline organic fraction within the pyrolysis product. Relative crystallinity is computed from the measured melting enthalpy normalized against the enthalpy of a 100% crystalline reference material. DSC also identifies residual rubber oligomers and waxes that may bias XRD results.
Complementary Raman Spectroscopy
First-order Raman spectra (D band near 1350 cm⁻¹, G band near 1580 cm⁻¹) provide a sensitive cross-check on carbon structural order. The ID/IG ratio and band deconvolution results correlate with the turbostratic-versus-ordered carbon balance inferred from XRD line-shape analysis.
Sample Preparation and Quality Control
All samples are milled, homogenized, and dried to constant mass. Ash content is determined gravimetrically (for example, per ASTM D1506 for carbon black materials) so that crystallinity can be reported both on an as-received basis and on a carbon-only basis. Internal standards such as corundum may be admixed for quantitative phase analysis. Each measurement batch includes calibration against a certified crystalline reference material, and results are reported with repeatability data.
Applications
Crystallinity data for lignin-rubber pyrolysis products directly support several industrial and research objectives:
- Development of lignin-based biofillers and partially bio-derived tire compounds with tailored reinforcing performance
- Quality control and grade classification of recovered carbon black produced by continuous or batch pyrolysis plants
- Optimization of pyrolysis temperature, residence time, and heating rate to maximize desirable carbon structural order
- Feasibility assessment of pyrolysis chars as precursors for activated carbon, energy storage carbons, and conductive additives
- Structure–property studies linking crystallinity to mechanical reinforcement, electrical conductivity, and adsorption capacity
- Regulatory documentation and circular-economy certification requiring structural characterization from accredited third-party laboratories
FAQ
Why is XRD preferred over DSC for crystallinity determination of pyrolysis chars?
Pyrolysis chars are predominantly infusible carbonaceous solids with no accessible melting transition. XRD directly quantifies the ratio of ordered (crystalline/turbostratic) to disordered (amorphous) scattering, making it the authoritative technique. DSC is retained only for residual semi-crystalline organic fractions.
How do inorganic residues affect the results?
Zinc oxide, zinc sulfide, silica, and calcium carbonate generate sharp diffraction peaks that can be misassigned to the carbon phase. Reputable protocols either demineralize the sample or perform multi-phase profile fitting with ash correction so that the reported crystallinity reflects the carbon fraction alone.
What sample quantity and form are required?
Typically 1–5 grams of milled, homogeneous powder is sufficient for a complete XRD-plus-DSC crystallinity package. Samples should be dry and free of oil contamination from the pyrolysis condensate stream.
What is a typical reporting format?
A standard report includes the diffraction pattern, deconvolution fit, crystallinity index and degree of crystallinity, d-spacings, crystallite sizes (Lc and La), DSC thermograms where applicable, ash-corrected values, and full identification of the calculation method (Segal, Ruland, or profile fitting).
What accuracy and turnaround can be expected?
Repeatability of ±2–3 percentage points in degree of crystallinity is typical for well-homogenized powders. Standard turnaround at accredited laboratories is approximately 5–10 working days, depending on whether demineralization or supplementary Raman analysis is required.