Experimental Study on the Effect of Pyrolysis Temperature on Lignin-Rubber Composites

The effect of pyrolysis temperature on lignin-rubber composites is a critical research topic for developing sustainable elastomer materials and valorizing biomass resources. This article presents a comprehensive experimental framework covering thermal decomposition behavior, kinetic analysis, evolved gas characterization, and mechanical performance evaluation of lignin-filled rubber systems across a temperature range of 200–800 °C. All testing protocols follow internationally recognized standards from ISO and ASTM, ensuring reproducible and comparable results for material scientists and rubber engineers.

Technical Overview

Lignin, the second most abundant natural polymer and a major byproduct of the pulp and bioethanol industries, has attracted significant attention as a renewable aromatic filler and reinforcing agent for rubber compounds. When lignin-rubber composites are subjected to thermal treatment or pyrolysis, temperature becomes the dominant variable controlling decomposition pathways, char formation, and the resulting material properties. The experimental investigation of pyrolysis temperature effects therefore provides essential data for both composite formulation design and thermochemical valorization processes.

The thermal degradation of lignin-rubber systems proceeds through several overlapping stages. Below 150 °C, absorbed moisture and low-molecular-weight volatiles are released. Between 200 and 400 °C, lignin undergoes primary depolymerization involving cleavage of β-O-4 ether linkages, demethoxylation, and side-chain scission, while natural rubber chains begin to depolymerize through cis-1,4-polyisoprene scission near 350–380 °C. Above 400 °C, secondary cracking of primary volatiles, dehydrogenation, condensation of aromatic structures, and char consolidation dominate, with the char skeleton continuing to densify up to 600–800 °C. Understanding how these stage transitions shift with temperature allows engineers to optimize cure cycles, predict service lifetime, and design controlled pyrolysis processes for recovering phenolic monomers, fuel oils, or porous carbons from lignin-rubber waste.

Experimental evidence consistently demonstrates that increasing pyrolysis temperature raises the fixed carbon and char residue yield while reducing volatile matter content, shifts the maximum mass-loss-rate peak to higher temperatures, and alters the relative proportions of evolved gases such as CO, CO₂, CH₄, and phenolic compounds. In composite applications, thermally pretreated lignin fillers exhibit progressively higher aromaticity and carbon content with increasing treatment temperature, which in turn modifies vulcanization kinetics, crosslink density, reinforcement efficiency, and dynamic mechanical response of the rubber matrix.

Testing Samples

Lignin Feedstock Samples

Commercial and technical lignins representative of industrial availability are collected and characterized before compounding, including at minimum the following grades:

• Kraft lignin precipitated from black liquor, washed and vacuum-dried at 60 °C for 24 hours.
• Alkali (soda) lignin from agricultural straw pulping, sieved to a particle size below 75 μm.
• Enzymatic hydrolysis lignin derived from cellulosic ethanol production, used as received after dehydration.

Thermally Treated Lignin Filler Series

To isolate the effect of thermal history on the filler phase, lignin samples are pretreated in a tubular furnace under high-purity nitrogen at a set of graduated temperatures, typically 200 °C, 300 °C, 400 °C, 500 °C, and 600 °C, each held for 60 minutes at a heating rate of 10 °C/min. The resulting series of carbonized fillers is stored in a desiccator before compounding to prevent moisture reabsorption.

Lignin-Rubber Composite Specimens

Composite specimens are prepared by melt compounding in an internal mixer followed by sulfur vulcanization in a hydraulic press. The experimental matrix covers:

• Rubber matrices: natural rubber (NR), styrene-butadiene rubber (SBR), butadiene rubber (BR), and ethylene-propylene-diene rubber (EPDM).
• Lignin loading levels: 10, 20, 30, 40, and 50 phr, with and without partial replacement of carbon black.
• Vulcanized sheets of 2 mm thickness for mechanical testing and finely cut granules of 0.5–1 mm for thermogravimetric and pyrolysis analysis.
• Reference compounds containing neat rubber and conventional carbon black filler for baseline comparison.

Testing Items

The experimental program quantifies temperature-dependent changes across thermal, chemical, rheological, mechanical, and morphological properties. Core testing items include:

• Thermal decomposition characteristics: onset degradation temperature (T_onset extrapolated), temperature of maximum mass-loss rate (T_max from DTG curves), and residual char yield at 600 °C and 800 °C.
• Kinetic parameters: apparent activation energy (Ea) and pre-exponential factor determined by Kissinger, Ozawa–Flynn–Wall, and Friedman isoconversional methods.
• Evolved gas and pyrolysis product composition: relative yields of CO, CO₂, CH₄, H₂O, light hydrocarbons, guaiacol-type phenols, and polycyclic aromatics.
• Functional group evolution: changes in hydroxyl, methoxy, carbonyl, and aromatic band intensities with treatment temperature.
• Vulcanization characteristics: minimum torque (ML), maximum torque (MH), delta torque, scorch time (ts2), optimum cure time (t90), and cure rate index (CRI).
• Mechanical properties: tensile strength, 100% and 300% modulus, elongation at break, tear strength, and Shore A hardness.
• Dynamic mechanical properties: storage modulus, loss modulus, and tan δ at 0 °C (wet grip indicator) and 60 °C (rolling resistance indicator).
• Morphological and surface characterization: filler dispersion, fracture surface topography, char microstructure, and specific surface area of pyrolyzed residues.

Testing Methods

Thermogravimetric Analysis (TGA/DTG)

Thermogravimetric analysis is performed according to ISO 11358 and ASTM E1131 using a simultaneous thermal analyzer equipped with a high-sensitivity microbalance (resolution 0.1 μg) and a temperature accuracy of ±0.5 °C. Samples of 5–10 mg are placed in open alumina pans and heated from ambient temperature to 800 °C under nitrogen purge at 60 mL/min. Multiple heating rates of 5, 10, and 20 °C/min are applied to generate the datasets required for isoconversional kinetic analysis. Derivative thermogravimetric (DTG) curves are computed to identify staged decomposition events and T_max values for each lignin grade and composite formulation.

Evolved Gas Analysis (TG-FTIR and Py-GC/MS)

Coupled thermogravimetry–Fourier transform infrared spectroscopy is used to monitor functional groups of evolved gases in real time, following the general spectroscopic practice of ASTM E1252. The transfer line and gas cell are maintained at 200 °C to prevent condensation, and spectra are recorded every few seconds across the full heating program. Flash pyrolysis–gas chromatography/mass spectrometry at set temperatures of 300 °C, 400 °C, 500 °C, and 600 °C provides semi-quantitative product distributions, distinguishing primary lignin depolymerization products from secondary cracking species such as benzene, toluene, and naphthalene formed at elevated temperatures.

Vulcanization Rheometry

Cure characteristics of the uncured compounds are measured with a moving-die rheometer in accordance with ASTM D5289 and ISO 6502 at 150 °C or 160 °C with a 0.5° arc amplitude. Comparison of torque parameters among fillers pretreated at different temperatures reveals how progressive carbonization of lignin affects accelerator adsorption, scorch safety, and ultimate crosslink density.

Mechanical and Dynamic Mechanical Testing

Tensile properties are determined on dumbbell specimens die-cut from vulcanized sheets according to ASTM D412 and ISO 37, using a universal testing machine with a 500 N load cell and a crosshead speed of 500 mm/min. Shore A hardness is measured per ASTM D2240 and ISO 48. Dynamic mechanical analysis is conducted in tension mode in line with ISO 6721-4 over a temperature sweep from −80 °C to 80 °C at 10 Hz to capture glass transition behavior and tan δ performance indicators relevant to tire applications. A minimum of five replicates are tested for each mechanical property, and results are reported as mean values with standard deviations.

Spectroscopic and Microscopic Characterization

Attenuated total reflectance FTIR spectroscopy tracks the disappearance of the methoxy band near 1260 cm⁻¹ and hydroxyl band near 3400 cm⁻¹ as pretreatment temperature rises, while Raman spectroscopy quantifies the D/G band intensity ratio of carbonized fillers. Fracture surfaces and pyrolysis chars are examined by scanning electron microscopy after sputter coating, and the specific surface area of chars is determined by nitrogen adsorption following the BET method in accordance with ISO 9277.

Quality Control and Reporting

All thermal experiments are run in triplicate, and blank correction curves are subtracted where buoyancy effects are significant. Instrument calibration is verified with indium and zinc melting point standards before each test series. The final test report documents sample identification, lignin grade and loading, pretreatment temperature, test atmosphere, heating rate, mass-loss curves, calculated kinetic parameters, and statistical scatter, providing a complete traceable record suitable for research publication and industrial qualification alike.

Applications

The experimental data generated from pyrolysis temperature studies directly supports several high-value engineering applications:

• Sustainable tire technology: temperature-optimized lignin fillers partially replacing carbon black and silica, reducing rolling resistance while maintaining wet grip and abrasion resistance.
• Bio-based industrial rubber goods: hoses, seals, conveyor belts, and anti-vibration mounts incorporating lignin-derived reinforcement with verified thermal stability.
• Waste valorization: co-pyrolysis of spent lignin-rubber products to recover phenolic chemicals, fuel oil, and syngas, with temperature data guiding process windows for maximum liquid yield.
• Activated carbon production: lignin-rubber chars produced at 500–700 °C serving as precursors for porous carbons used in adsorption and energy storage.
• Flame-retardant formulations: the high char-forming tendency of lignin exploited to improve the fire performance of elastomeric compounds.
• Life-cycle assessment and certification: standardized thermal degradation data supporting environmental declarations and compliance documentation prepared with accredited third-party laboratories.

FAQ

Why is pyrolysis temperature the most influential variable in lignin-rubber experiments? Temperature controls the sequence and extent of bond cleavage reactions. Between 200 and 400 °C, ether linkages and methoxy groups in lignin break down; above 400 °C, aromatic condensation and char consolidation dominate. Small changes in temperature therefore produce large differences in volatile yield, char structure, and residual filler reinforcement capability.

At what temperature does lignin-rubber composite degradation reach its maximum rate? Most natural rubber-based composites exhibit a dominant DTG peak between 360 and 390 °C, while lignin contributes a broad, thermally resistant shoulder extending from 200 °C to beyond 500 °C. The composite T_max typically shifts upward as lignin content increases, reflecting the stabilizing char barrier effect.

How does pyrolysis temperature affect char residue? Char yield increases progressively with final pyrolysis temperature, generally stabilizing above 600 °C once devolatilization is essentially complete. Composites containing 30–50 phr lignin commonly show residual char several times higher than neat rubber at 800 °C.

Does lignin accelerate or retard rubber vulcanization? Untreated lignin tends to adsorb accelerators and slightly retard cure, increasing scorch time and t90. As pretreatment temperature rises toward 500–600 °C, the filler becomes more graphitic and less polar, reducing accelerator adsorption and shifting cure behavior closer to that of carbon black.

Which standards govern these experiments? Thermal analysis follows ISO 11358 and ASTM E1131; tensile testing follows ASTM D412 and ISO 37; hardness follows ASTM D2240 and ISO 48; cure rheometry follows ASTM D5289 and ISO 6502; dynamic mechanical analysis follows ISO 6721; and BET surface area determination follows ISO 9277.

What sample mass and atmosphere are recommended for reliable TGA results? A sample mass of 5–10 mg with a nitrogen flow of approximately 60 mL/min is recommended to minimize heat and mass transfer limitations, ensure uniform degradation, and permit accurate kinetic evaluation at multiple heating rates.