Pore Structure Analysis of Styrene-Butadiene Rubber Pyrolysis Products

Pore structure analysis of styrene-butadiene rubber (SBR) pyrolysis products is essential for evaluating the quality, performance, and application potential of pyrolysis char and recovered carbon black derived from waste rubber and scrap tires. This technical article explains the key testing samples, measurable pore parameters, analytical methods such as BET nitrogen adsorption and DFT modeling, and practical applications of the results. Understanding these characterization techniques enables researchers and manufacturers to optimize pyrolysis conditions and unlock higher-value uses for SBR-derived carbon materials.

Technical Overview

Styrene-butadiene rubber (SBR) is one of the most widely consumed synthetic rubbers, primarily used in tire treads, conveyor belts, footwear soles, and various molded rubber goods. When SBR-based waste materials undergo pyrolysis, the polymer chains decompose into oil and gas fractions while the carbonaceous skeleton, together with embedded fillers and inorganic additives, remains as a solid residue known as pyrolysis char or recovered carbon black (rCB). The commercial and technical value of this residue depends heavily on its pore structure, including specific surface area, pore volume, pore size distribution, and pore morphology.

During pyrolysis, volatile matter escapes from the decomposing rubber matrix, leaving behind a network of microcracks, micropores, mesopores, and macropores. The development of this pore network is governed by several processing parameters, including pyrolysis temperature, heating rate, residence time, reactor pressure, and atmosphere. Low pyrolysis temperatures typically produce chars with partially blocked or collapsed pores due to residual tar deposits, whereas higher temperatures and adequate volatile release promote the opening and expansion of pores. Demineralization, activation with steam or CO2, and chemical activation with KOH or ZnCl2 can further modify the pore structure dramatically.

Accurate pore structure analysis provides critical data for process optimization, quality control of recovered carbon black, and the development of SBR-derived porous carbons for adsorption, energy storage, and environmental applications. Gas adsorption porosimetry, particularly nitrogen physisorption at 77 K, remains the industry-standard approach for this characterization.

Testing Samples

Raw SBR Pyrolysis Char

This category includes the as-produced solid residue obtained directly from pyrolysis of pure SBR, SBR compounds, or whole scrap tires at typical operating temperatures of 400–700 degrees Celsius. The char often contains original carbon black filler, inorganic ash from zinc oxide, silica, calcium carbonate, and sulfur-containing residues, all of which influence the measured pore parameters.

Recovered Carbon Black (rCB)

Recovered carbon black refers to the refined pyrolysis char after milling, pelletizing, and removal of coarse impurities such as steel wire and textile fibers. Pore structure analysis of rCB is essential for benchmarking its reinforcing performance against commercial carbon black grades such as N330, N550, or N660, and for determining its suitability as a replacement filler in new rubber compounds.

Activated Carbons Derived from SBR Char

Pyrolysis char can be converted into high-surface-area activated carbon through physical activation using steam or carbon dioxide at 800–950 degrees Celsius, or through chemical activation using potassium hydroxide, zinc chloride, or phosphoric acid. These samples frequently exhibit BET surface areas exceeding 500–2000 square meters per gram, requiring careful selection of analytical models and measurement conditions.

Upgraded and Demineralized Samples

Acid or base washing treatments remove inorganic ash components from the char, altering both the accessible pore volume and the surface chemistry. Pore structure analysis before and after demineralization helps quantify the contribution of mineral phases and reveals pores that were previously blocked by inorganic matter.

Testing Items

Comprehensive pore structure characterization of SBR pyrolysis products typically covers the following measurable parameters:

• BET specific surface area, calculated from nitrogen adsorption data over the relative pressure range of 0.05 to 0.30
• Langmuir specific surface area, applicable to predominantly microporous samples
• Micropore surface area and external surface area, determined by the t-plot method
• Total pore volume, calculated from the amount of nitrogen adsorbed at a relative pressure near 0.99
• Micropore volume, obtained from t-plot or Dubinin-Radushkevich analysis
• Mesopore volume and distribution, evaluated using the BJH (Barrett-Joyner-Halenda) method
• Micropore size distribution, calculated using the Horvath-Kawazoe, Saito-Foley, or density functional theory (DFT) models
• Average pore diameter, derived from the surface area and total pore volume assuming a cylindrical or slit-shaped pore geometry
• Complete adsorption-desorption isotherms, including hysteresis loop classification according to IUPAC conventions

Testing Methods

Sample Pretreatment and Degassing

Before measurement, samples are degassed under vacuum or flowing gas to remove physically adsorbed moisture and contaminants from the pore network. Typical degassing conditions involve heating at 150–300 degrees Celsius for 6 to 12 hours. Because SBR-derived chars often retain tarry residues and polar surface groups, selecting an adequately high yet non-destructive degassing temperature is critical for accurate and reproducible results.

Nitrogen Physisorption at 77 K

The sample is cooled to liquid nitrogen temperature, and the quantity of nitrogen adsorbed is measured at a series of precisely controlled relative pressures to construct the adsorption isotherm. Desorption data are subsequently collected to identify hysteresis behavior. SBR pyrolysis chars commonly display Type II or Type IV(a) isotherms with H3 or H4 hysteresis loops, indicating the coexistence of slit-shaped mesopores, interparticle voids, and a microporous carbonaceous framework.

Carbon Dioxide Adsorption at 273 K

Nitrogen at 77 K experiences activated diffusion limitations in ultramicropores narrower than approximately 0.7 nanometers. Carbon dioxide adsorption at 273 K, analyzed with appropriate DFT or Dubinin models, provides a more reliable assessment of the narrow micropore volume and ultramicropore size distribution, which is particularly valuable for activated SBR-derived carbons intended for gas separation or storage.

Data Analysis and Model Selection

BET analysis is applied to confirm the linear region and positive C constant before reporting surface area. For micro-mesoporous samples, non-local density functional theory (NLDFT) or quenched solid density functional theory (QSDFT) provides the most physically consistent pore size distribution across the combined micro- and mesopore range. BJH analysis, while widely used, tends to underestimate pore sizes below approximately 10 nanometers and should be interpreted accordingly.

Complementary Techniques

Mercury intrusion porosimetry characterizes macropores and larger mesopores above roughly 3.6 nanometers, including the interparticle voids typical of pelletized recovered carbon black. Scanning electron microscopy and transmission electron microscopy reveal pore morphology and the degree of filler particle fusion, while Raman spectroscopy and X-ray diffraction assess graphitic ordering, which correlates with pore wall structure and adsorption energetics.

Applications

Pore structure data from SBR pyrolysis products support a wide range of industrial and research objectives:

• Optimization of tire pyrolysis reactor conditions, including temperature, residence time, and heating rate, to maximize char porosity and quality
• Quality control and grading of recovered carbon black for reuse as a reinforcing filler in new tire and rubber products, supporting circular economy initiatives
• Development of low-cost activated carbons for wastewater treatment, dye adsorption, and removal of heavy metal ions
• Design of adsorbents for gas storage, volatile organic compound capture, and post-combustion CO2 capture
• Fabrication of electrode materials for supercapacitors and lithium-ion or sodium-ion battery anodes, where high micropore surface area directly enhances charge storage
• Catalyst and catalyst support development, where controlled mesoporosity improves mass transport and active site accessibility
• Academic research into the mechanistic relationship between SBR decomposition chemistry and carbon pore evolution

FAQ

How does pyrolysis temperature affect the pore structure of SBR-derived char?

Increasing pyrolysis temperature generally releases more volatile matter, opening previously blocked pores and raising surface area up to an optimum, often between 500 and 600 degrees Celsius. Excessively high temperatures can cause pore wall sintering, graphitization, and partial pore collapse, which reduce the measured surface area and total pore volume.

Why is degassing necessary before measurement?

Adsorbed water, oil vapors, and atmospheric gases occupy pore surfaces and prevent adsorbate molecules from accessing the pore network. Incomplete degassing leads to underestimated surface areas and distorted pore size distributions, which is especially problematic for tar-containing pyrolysis chars.

Which pore size distribution model should be used for SBR pyrolysis products?

NLDFT or QSDFT models are recommended for carbonaceous materials containing both micropores and mesopores, because they account for the heterogeneous surface geometry of carbon. BJH is acceptable for comparative mesopore analysis, while Horvath-Kawazoe or CO2-based DFT analysis is preferred for the narrow micropore region.

What are typical BET surface area values for these materials?

Raw SBR pyrolysis char and as-produced recovered carbon black usually exhibit BET surface areas between 40 and 100 square meters per gram, largely governed by the original carbon black filler. Physically or chemically activated derivatives can reach 500 to 2000 square meters per gram or higher, depending on the activation severity.

How does inorganic ash content influence the results?

Ash components such as zinc oxide, silica, and calcium carbonate dilute the carbonaceous fraction and may block pore entrances, lowering the apparent surface area and pore volume. Reporting results on both an as-received and ash-free basis improves comparability between samples and production batches.

Can nitrogen and carbon dioxide adsorption results be compared directly?

Not directly. The two techniques probe different temperature regimes and pore width ranges. CO2 adsorption at 273 K accesses ultramicropores more effectively, so CO2-derived surface areas are often higher than N2-derived values for strongly microporous samples. Both measurements are complementary and are frequently reported together for a complete pore structure picture.