Crystal Phase Structure Analysis of Styrene-Butadiene Rubber Pyrolysis Products

Crystal phase structure analysis of styrene-butadiene rubber (SBR) pyrolysis products reveals the mineralogical composition, crystallinity, and carbon structural order that determine the quality and end-use value of recovered carbon black and pyrolysis chars. Using advanced X-ray diffraction, Raman spectroscopy, and electron microscopy, our laboratory delivers accurate phase identification, quantitative phase composition, and crystallite size data for rubber recycling research and industrial process optimization. This article presents the technical background, sample types, testing items, analytical methods, and applications of SBR pyrolysis product crystal phase analysis.

Technical Overview

Styrene-butadiene rubber (SBR) is the most widely consumed synthetic elastomer in the world and the principal polymer in passenger tire treads and numerous industrial rubber goods. Pyrolysis, the thermal degradation of rubber in an oxygen-free atmosphere at approximately 400 to 700 degrees Celsius, converts waste SBR products into three product streams: condensable pyrolysis oil, non-condensable fuel gas, and a solid residue. The solid residue is a complex multiphase composite whose structure directly determines its commercial value.

The solid pyrolysis product of SBR-based articles typically consists of the original carbon black filler, a coating of pyrolytic carbon deposited onto the filler surface, and a series of crystalline inorganic phases derived from compounding additives. Zinc oxide, the standard vulcanization activator, is progressively converted into zinc sulfide through reaction with hydrogen sulfide released from decomposing sulfur crosslinks. Additional crystalline phases frequently include quartz and amorphous silica from fillers, calcite, calcium oxide formed at elevated temperatures, sulfates, and iron oxides from steel wire residues. The relative amounts and structural characteristics of these phases are sensitive functions of pyrolysis temperature, heating rate, residence time, and feedstock formulation.

Crystal phase structure analysis provides the qualitative and quantitative mineralogical information required to characterize this material: which crystalline phases are present, in what proportions, with what crystallite sizes, lattice parameters, and degrees of ordering, and how the carbonaceous fraction is organized from amorphous to turbostratic and partially graphitic states. These data underpin quality control of recovered carbon black, optimization of pyrolysis conditions, and the development of value-added applications for pyrolysis residues.

Testing Samples

Model SBR Pyrolysis Residues

Chars produced in laboratory-scale pyrolysis of virgin emulsion-polymerized or solution-polymerized SBR under strictly controlled temperature, heating rate, and residence time. These model samples establish the baseline phase evolution of polymer-derived carbon without interference from compounding ingredients.

Waste Tire Pyrolysis Char

Solid residues from industrial waste tire pyrolysis, containing SBR-derived pyrolytic carbon, the original commercial carbon black filler, ZnO and ZnS, silica, calcium carbonate, and trace metallic contaminants. Samples may be supplied as raw char, milled powder, or pelletized material.

Recovered Carbon Black (rCB)

Post-processed recovered carbon black products, including demineralized, acid-leached, surface-treated, or pelletized grades, analyzed for conformity with product specifications and batch-to-batch consistency.

Ash and Mineral-Enriched Fractions

Combustion ash, acid-digested residues, or density-separated fractions in which the inorganic phases are concentrated, enabling detailed characterization of minor crystalline components that are masked in bulk samples.

Testing Items

• Qualitative identification of crystalline phases, including ZnO (wurtzite), ZnS (sphalerite or wurtzite), SiO2, CaCO3, CaO, CaS, sulfates, iron oxides, and accessory minerals.
• Quantitative phase composition determined by Rietveld refinement or reference-intensity-ratio methods.
• Amorphous phase content, including the disordered carbon fraction, quantified by the internal-standard XRD method.
• Crystallite size and microstrain of individual phases calculated by the Scherrer equation and Williamson-Hall analysis.
• Lattice parameters, unit-cell dimensions, and characteristic d-spacings, including the interlayer spacing (d002) of the carbonaceous phase.
• Degree of carbon structural ordering, including stacking height (Lc), lateral crystallite size (La), turbostratic character, and graphitization degree.
• Zinc phase speciation, expressed as the ZnO/ZnS ratio, as a diagnostic indicator of desulfurization extent and pyrolysis severity.
• In-situ variable-temperature XRD tracking of phase transformations such as ZnO-to-ZnS conversion and CaCO3 decomposition.

Testing Methods

X-Ray Diffraction (XRD)

Powder XRD is the primary technique for crystal phase analysis. Measurements are performed on a powder diffractometer with Cu K-alpha radiation (wavelength 1.5406 angstroms) in Bragg-Brentano geometry, typically scanning from 5 to 90 degrees two-theta with a step size of 0.02 degrees. Phase identification is carried out against the ICDD PDF database, and quantitative phase analysis is achieved through full-pattern Rietveld refinement using software such as GSAS, FullProf, or TOPAS. A corundum internal standard is employed for amorphous content quantification. Samples are ground below 45 micrometers and prepared by back-loading to minimize preferred orientation.

Raman Spectroscopy

Raman spectroscopy is complementary to XRD for the carbonaceous fraction, which is largely disordered. The disorder-induced D band near 1350 cm-1 and the graphitic G band near 1580 cm-1 are analyzed; the ID/IG intensity ratio quantifies defect density and structural ordering, while the 2D band shape distinguishes turbostratic from graphitic stacking. Raman also confirms nanocrystalline ZnO and ZnS through their characteristic phonon modes.

High-Resolution TEM and SAED

High-resolution transmission electron microscopy provides direct imaging of carbon layer fringes and embedded nanocrystalline inclusions, along with fringe-statistics analysis of local structural order. Selected-area electron diffraction ring patterns verify nanocrystalline phases at the nanometer scale.

SEM-EDS

Scanning electron microscopy with energy-dispersive X-ray spectroscopy characterizes particle morphology and provides element distribution maps of Zn, S, Si, Ca, and Fe, correlating elemental and crystallographic information within individual particles.

Supporting Thermal and Spectroscopic Techniques

Thermogravimetric analysis per ASTM E1131 determines volatile, carbon, and ash fractions that constrain the XRD mass balance. FTIR identifies surface functional groups, XPS provides sulfur and zinc surface speciation, and nitrogen adsorption per ASTM D6556 supplies surface area data for correlation with structural parameters. Testing frameworks for recovered carbon black follow the guidance of ASTM D8181.

Applications

• Quality control and certification of recovered carbon black for reuse in tire and general rubber compounding.
• Pyrolysis process optimization, correlating reactor temperature, residence time, and atmosphere with ZnS formation, CaCO3 decomposition, and carbon ordering.
• Development of value-added carbon materials from pyrolysis residues, including adsorbents, catalyst supports, pigments, and battery electrode materials.
• Rubber compound formulation studies that require knowledge of inorganic phase loading and its effect on cure behavior and mechanical properties.
• Environmental assessment of pyrolysis residues, including heavy-metal speciation relevant to leaching behavior, disposal classification, and regulatory compliance.
• Academic research into elastomer degradation mechanisms, carbonization kinetics, and polymer-to-carbon conversion pathways.

FAQ

Why does zinc sulfide often dominate the crystalline phases in SBR pyrolysis residue?

During pyrolysis, sulfur crosslinks in the vulcanized network decompose and release hydrogen sulfide. The H2S reacts with the zinc oxide vulcanization activator to form ZnS, which crystallizes as sphalerite and, at higher temperatures, wurtzite. The ZnO/ZnS ratio therefore serves as a built-in indicator of pyrolysis severity and desulfurization extent.

Can XRD quantify the amorphous carbon fraction?

Yes. By blending the sample with a known mass of a crystalline internal standard such as corundum and performing full-pattern refinement, the amorphous content is obtained from the intensity deficit of the standard phase. The result is typically cross-validated with TGA ash data and Raman spectroscopy.

How does pyrolysis temperature influence the crystal phase structure?

Between 400 and 500 degrees Celsius, carbonization of the rubber begins and ZnO starts converting to ZnS. From 500 to 700 degrees Celsius, ZnS becomes the dominant zinc phase, calcite remains stable, and carbon ordering improves gradually. Above approximately 750 to 800 degrees Celsius, CaCO3 decomposes to CaO, and the carbonaceous phase shows increased stacking order and crystallite growth.

How is the degree of graphitization assessed?

Three complementary parameters are used: the interlayer spacing d002 from XRD, which is about 3.354 angstroms for ideal graphite versus 3.44 to 3.60 angstroms for turbostratic carbon; the crystallite dimensions Lc and La derived from diffraction peak widths; and the Raman ID/IG ratio. Together these parameters describe the transition from amorphous char toward ordered graphitic structure.

What sample quantity and form are required for testing?

Approximately 0.5 to 2 grams of fine powder, preferably ground below 45 micrometers, is sufficient for a full XRD analysis including quantitative refinement. Raman and SEM-EDS require only milligram quantities, while TEM examination needs a few milligrams of dispersed powder. Pelletized samples should be ground before submission.

Our laboratory combines rigorous sample preparation, state-of-the-art instrumentation, and experienced crystallographic interpretation to deliver reliable crystal phase structure data for SBR pyrolysis products. Contact our technical team to discuss your specific analytical requirements and receive a customized testing plan.