Determination of the thermal conductivity of styrene-butadiene rubber (SBR) pyrolysis products provides essential heat-transfer data for reactor design, process simulation, and quality control in the rubber recycling and pyrolysis industries. This technical article describes the relevant sample types, measurable parameters, recognized testing methods and standards, and practical industrial applications for characterizing the thermal transport properties of SBR-derived char, pyrolysis oil, and recovered carbon black.
Technical Overview
Styrene-butadiene rubber (SBR) is one of the most widely consumed synthetic elastomers, used extensively in tire treads, conveyor belts, footwear soles, and molded rubber goods. When SBR-based waste materials such as scrap tires are heated under oxygen-free or oxygen-limited conditions, they undergo thermal decomposition (pyrolysis) and yield three principal product streams: non-condensable gas, condensable pyrolysis oil, and a solid carbonaceous residue known as char or recovered carbon black (rCB). The thermal conductivity of these products governs heat transfer inside the pyrolysis reactor, controls heating and cooling behavior in downstream handling, and directly influences product performance in compounding, insulation, and fuel applications.
Accurate thermal conductivity determination is therefore essential for pyrolysis kinetic modeling, reactor scale-up, energy balance calculations, and commercial certification of pyrolysis products. Because SBR pyrolysis products exist as gases, liquids, powders, porous solids, and pellets, a combination of transient and steady-state measurement techniques is typically applied to cover the full range of material states and operating temperatures encountered in industrial practice.
Testing Samples
Samples submitted for thermal conductivity determination generally fall into four categories, each requiring specific conditioning and preparation before measurement.
Pyrolysis Char and Solid Residue
The solid residue remaining after SBR pyrolysis consists of carbonaceous char, inorganic ash, original fillers such as carbon black or silica previously compounded into the rubber, and possible traces of steel wire. Before testing, the residue is typically ground, sieved to a controlled particle size distribution (commonly below 150 μm), and dried at approximately 105 °C to constant mass to eliminate moisture and residual volatile matter that would otherwise distort the measurement.
Recovered Carbon Black (rCB)
Commercially produced recovered carbon black may be supplied as loose powder or as densified pellets. Because interparticle air gaps strongly suppress effective conductivity in loose powders, rCB is frequently characterized both in its as-received powder state and in pelletized form. Pellet density, porosity, and ash content are recorded alongside thermal data, since these parameters correlate directly with the measured thermal conductivity.
Pyrolysis Oil and Condensable Fractions
SBR pyrolysis oil is a complex mixture of aromatics, olefins, and light hydrocarbons. Prior to measurement, the oil is filtered to remove suspended carbon fines, homogenized, and degassed under vacuum. Water content is determined by Karl Fischer titration, and measurements are performed in sealed cells to prevent evaporation and oxidative degradation during heating.
Molded and Compounded Specimens
Where pyrolysis char or rCB is used as a filler in new rubber or polymer compounds, test specimens are compression-molded into flat discs or plates. Typical geometries include discs 12.7 mm in diameter and 2 to 3 mm thick for laser flash analysis, or twin plates of at least 30 mm square for transient plane source testing.
Testing Items
Core measurement deliverables for a thermal characterization program typically include:
- Thermal conductivity (λ), reported in W/(m·K) at specified temperature points
- Thermal diffusivity (α), reported in mm²/s
- Specific heat capacity (cp), reported in J/(g·K), usually measured by differential scanning calorimetry
- Bulk density and true (skeletal) density of powders, pellets, and molded specimens
- Temperature-dependent thermal conductivity curves, commonly from room temperature to 300 °C, 500 °C, or higher
- Effective thermal conductivity of packed powder beds as a function of bulk density and compaction pressure
- Correlation reports linking conductivity with proximate analysis data such as ash content, volatile matter, and fixed carbon
Testing Methods
Transient Plane Source (Hot Disk) Method, ISO 22007-2
A double-spiral nickel sensor sandwiched between two sample halves acts simultaneously as a constant heat source and a resistance thermometer. From the temperature rise response, thermal conductivity and diffusivity are extracted in a single measurement. The method covers a wide conductivity range (approximately 0.005 to 500 W/(m·K)), accommodates solids, powders in holder cells, and viscous liquids, and operates from sub-ambient temperatures up to approximately 500 °C with appropriate furnaces. Typical reproducibility is within 3 to 5 percent.
Laser Flash Analysis, ASTM E1461 and ISO 22007-4
A short laser pulse irradiates the front face of a disc-shaped specimen, and an infrared detector records the temperature evolution of the rear face to yield thermal diffusivity directly. When combined with independently measured specific heat capacity and density, thermal conductivity is calculated from the relationship λ = α × ρ × cp. This method is well suited to dense char, graphitized residues, and molded compounds at temperatures up to and beyond 1000 °C.
Transient Hot Wire Method for Liquids, ASTM D7896
A thin heated wire immersed in the fluid serves as the heat source and temperature sensor. Because the measurement is completed within seconds, convection errors are minimized. The technique is the preferred approach for SBR pyrolysis oil and heavy condensate fractions, delivering accuracy on the order of 2 to 3 percent across temperatures from approximately -50 °C to 200 °C.
Guarded Hot Plate and Heat Flow Meter, ASTM C177 and ASTM C518
These steady-state methods establish a one-dimensional heat flux through a slab specimen between temperature-controlled plates. They are appropriate for low-conductivity char-based insulation boards, porous monoliths, and larger molded panels where through-thickness conductivity is the parameter of interest.
Sample Preparation and Data Reporting
All specimens are conditioned to a defined dry state, weighed, and dimensionally verified. Powder samples are loaded into calibrated holder cells at documented tap densities. Each reported value represents the mean of at least three valid measurements, and final reports include the test standard, sensor geometry, temperature points, density basis, and uncertainty statements so that results are traceable and comparable between laboratories.
Applications
- Pyrolysis reactor design and CFD simulation, where reliable conductivity values for feedstock, char, and oil determine heat-up predictions and residence time calculations
- Waste tire recycling process optimization, including energy balance auditing of continuous and batch pyrolysis plants
- Quality control of recovered carbon black intended for reuse as a reinforcing filler in new rubber compounds
- Heat exchanger, storage tank, and piping design for facilities handling pyrolysis oil as an industrial fuel
- Development of char-based insulation and construction materials with engineered thermal performance
- Academic and industrial research on pyrolysis kinetics, heat transfer mechanisms, and structure-property relationships in carbonaceous residues
FAQ
What is the typical thermal conductivity range of SBR pyrolysis products?
Pyrolysis oil generally falls between 0.09 and 0.15 W/(m·K) near room temperature, while loose char and rCB powder beds typically measure from roughly 0.05 to 0.2 W/(m·K) because of entrained air. Densified pellets and molded compounds can reach 0.3 W/(m·K) or higher, and residues that have undergone high-temperature graphitization can exceed these values considerably.
At what temperatures can measurements be performed?
Standard service conditions commonly cover room temperature to 500 °C using transient plane source equipment, and room temperature to 1000 °C or above using laser flash analysis. Liquid measurements with the transient hot wire method are usually limited to approximately 200 °C to keep the oil stable.
How much sample is required for testing?
For hot disk testing, two matching pieces of at least 30 mm across and a few millimeters thick, or several grams of powder, are sufficient. Laser flash analysis requires a single disc approximately 12.7 mm in diameter and 2 to 3 mm thick. Liquid testing needs approximately 40 to 100 mL depending on the cell configuration.
How are powders such as char and recovered carbon black tested?
Powders are placed in dedicated holder cells that fix the sensor position, and measurements are performed at documented bulk densities. Testing at several compaction levels allows the laboratory to construct a conductivity-versus-density curve, which is valuable for reactor modeling of packed beds.
Which standard should be selected for a given sample?
ISO 22007-2 (hot disk) is the most versatile choice for solids, powders, and pastes; ASTM E1461 (laser flash) is preferred for dense solids at elevated temperatures; ASTM D7896 (transient hot wire) is recommended for pyrolysis oils; and ASTM C177 or ASTM C518 applies to insulation-type slab materials.